Content addressable memory

ABSTRACT

An entry including multiple bits of unit cells each storing data bit is coupled to a match line. The match line is supplied with a charging current having a restricted current value smaller than a match line current flowing in a one-bit miss state in one entry, but larger than a match line current flowing in an all-bit match state in one entry. A precharge voltage level of a match line is restricted to a voltage level of half a power supply voltage or smaller. Power consumption in a search cycle of a content addressable memory can be reduced, and a search operation speed can be increased.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a content addressable memory, andparticularly to a construction for reducing current consumption and apeak current in a search operation and for speeding up the searchoperation.

2. Description of the Background Art

A CAM (Content Addressable Memory) has, in addition to a function ofreading/writing data, a function of determining matching of stored datawith supplied search data. One entry storing a search data word isconfigured by a plurality of CAM cells, and stores word bits of a searchcandidate. Each entry is provided with a match line coupled tocorresponding CAM cells in parallel. When a search data word matches astored data word, a corresponding match line is kept at a state of “1”.When mismatch occurs, the corresponding match line is driven to a stateof “0”.

By determining the voltage level of the match line, it is possible todetermine whether the data corresponding to the search data is stored,e.g., in a table. This type of content addressable memory is used fordetermining cache-miss/hit in a cache memory and a router forcommunications application. A network router or the like performsrouting of an IP packet through comparison of an IP address stored in acontent addressable memory provided in the router with an externallysupplied IP address. For example, a value indicating a next destinationaddress is written into an IP packet based on match line information ofthe matching state in the content addressable memory in the router, andthen the IP packet is delivered from a corresponding port.

Usually, in the CAM used in the communications router or the like,search data has a bit width from 72 bits to 288 bits, and the number ofthe entries is about 64 K.

In a conventional CAM, the match lines are precharged to a power supplyvoltage VDD (or a ground voltage GND level) during a precharge period.During a search period for detecting matching between the stored dataand the search data, the search data is compared with data bits of entryCAM cells. When mismatch occurs, transistors in the CAM cells discharge(or charge) the corresponding match line to the ground voltage (or powersupply voltage level) different from the precharge voltage. Therefore,when n CAM cells are in the mismatch state in one entry, a current of(I_miss×n) is discharged (or charged) through one match line, whereI_miss represents a current driven by one CAM cell in the mismatchstate. When the match occurs in all the data bits of the CAM cells inthe entry, no discharge (or charge) path is present in the CAM cells.Therefore, the match line in the match state is kept at the level of theprecharge voltage of power supply voltage VDD or ground voltage GND.

In the CAM, the search data is supplied in parallel to the plurality ofentries, and the search operations are executed in parallel in therespective entries. Search line transmitting the search data and thematch line producing a signal indicative of a match result areprecharged to predetermined voltage levels in each search cycle ofperforming the search operation. As an example, the search line isprecharged to the ground voltage level, and the match line is prechargedto the power supply voltage level. The search line is coupled to the CAMcells in all entries. Therefore, the search line has a large capacitiveload. A majority of the match lines except the line exhibiting the matchstate in the search operation change between the power supply voltagelevel and the ground voltage level in each search cycle. Therefore, thecharge and discharge currents of the search line and the match line arelarge, resulting in a problem that current dissipation and powerconsumption are large.

An article 1 (H. Noda, et al., “A Cost-Efficient High-PerformanceDynamic TCAM With Pipelined Hierarchical Searching and Shift RedundancyArchitecture”, IEEE JSSC, Vol. 40, No. 1, January 2005, pp. 245-253) hasdisclosed a construction for reducing current consumption of a contentaddressable memory and achieving a fast search operation. In theconstruction disclosed in the article 1, the match line has ahierarchical configuration. A plurality of local match lines arearranged for one entry. These plurality of local match lines arecommonly coupled to a global match line. Searching operation isperformed for each local match line in a pipelined fashion. For example,search data of 144 bits is divided into 72-bit data. On the local matchline of the block in which no matching occurs in first 72 bits, it is nolonger necessary to perform the subsequent search. Therefore, in theentries corresponding to the mismatch block, the search line is notactivated in the next stage, and the local match lines are notdischarged. The number of the local match lines to be charged anddischarged can be reduced, and the power consumption can be reduced.

In the article 1, a DRAM-type cell construction is employed for storingsearch data, and each DRAM cell stores a data bit, to store ternarydata. The CAM cell storing the ternary data is generally referred to asa TCAM (Ternary CAM) cell, and can store a “don't care” state.

A publication 1 (Japanese Patent Laying-Open No. 10-027481) hasdisclosed a construction aiming to achieve a fast search operation. Inthe construction disclosed in the publication 1, a match line isprecharged to a ground voltage level during standby. In a searchoperation, each match line is supplied with a current of a magnitudesubstantially equal to that flowing when one-bit mismatch occurs withrespect to the search data. Voltage rising of the match line in themismatch entry is suppressed to or below a reference voltage forreducing the current consumption.

A publication 2 (Japanese Patent Laying-Open No. 2004-192695) haslikewise disclosed a construction aiming to reduce the currentconsumption in a search operation. In the publication 2, complementarysearch lines are short-circuited to precharge the search lines to anintermediate voltage level during standby. A match line is precharged tothe ground voltage level during standby, and is charged up byaccumulated charges supplied from a capacitance element when the searchoperation starts. A capacitive division by the capacitance element andthe match line sets an upper limit of the voltage level of the matchline to an intermediate voltage level lower than the power supplyvoltage. A buffer circuit is employed for sensing the voltage level ofthe match line.

A publication (Japanese Patent Laying-Open No. 2003-100086) hasdisclosed a construction for executing a fast search operation even whena match line load is large. In the publication 3, a reference voltagegenerating circuit and a differential amplifier circuit are arranged foreach match line. The differential amplifier circuit compares a referencevoltage with a match line voltage for increasing a speed of a search anddetermination operation.

A publication 4 (Japanese Patent Laying-Open No. 2002-358791) hasdisclosed a construction for reducing a precharge current in a searchoperation. In the publication 4, CAM entries are divided. For thedivided CAM entries, precharge voltages and the drive voltage of thematch line in the miss state are set oppositely. Specifically, on onedivided entry, the match line is precharged to an H level (logical higHlevel), and is set to an L level (logical low level) in the mismatch. Onanother divided entry, the match line is precharged to the L level, andis set to the H level in the mismatch. By short-circuiting the matchlines in the divided CAM entries, redistribution of electric chargesoccurs in the mismatch entry during precharging, and the divided matchlines therein are driven to an intermediate voltage, so that the currentconsumption is reduced.

A publication 5 (Japanese Patent Laying-Open No. 2002-245783) has alsodisclosed a construction for reducing current consumption in a searchoperation. In the construction disclosed in the publication 5, a dummymatch line is provided to have the same capacitance as an entry in amatch state. A match line and the dummy match line are precharged to aground voltage, and are supplied with a current in a search operation.When the dummy match line is determined to be at the voltage level of anH level, a determination timing signal is produced to stop charging ofthe match line. Current consumption is reduced by reducing a period forsupplying the current to the match line. A differential amplifiercircuit is used for determining the voltage level of the match line, andcompares a reference voltage with the match line voltage.

A publication 6 (Japanese Patent Laying-Open No. 2001-319481) hasdisclosed a construction for reducing current consumption in a searchoperation and for increasing a speed of the search operation. In theconstruction disclosed in the publication 6, a bit line forwriting/reading data is provided separately from a search line fortransferring search data. The bit line is precharged to the H level, andthe search line is precharged to the L level. In a search operation, thesearch and bit lines are short-circuited according to the search data sothat the search line at a high level is set to an intermediate voltagelevel, and an amplitude of a search line voltage is set to be betweenthe ground voltage and the intermediate voltage level. The match line isprecharged to the intermediate voltage level, and is charged up via adecouple transistor in the search operation. Via such decoupletransistor, the match line is coupled to a sense amplifier. Even when asense node of input nodes of the sense amplifier is charged, thedecouple transistor suppresses voltage rising of the match line. Uponmismatch, the sense node is discharged via the match line. The voltageamplitude(s) of the match line and/or search line is (are) restricted,so that the current consumption is reduced, and the search operation isspeeded up.

In the CAM and TCAM, as described above, the search line and the matchline are charged and discharged in each search cycle, and the currentconsumption is large. In the article 1 described above, the match linehas a hierarchical configuration, and the search operation is performedin a pipelined fashion for each of the plurality of local match lineblocks. For the entry of the mismatching in a certain pipeline stage(local match line block), the subsequent discharging of the search lineand the local match line is not performed for reducing the currentconsumption.

In the article 1, the match line has the hierarchical configuration, butthe search line is arranged commonly to all the entries. Therefore, thesearch line of a large load capacitance is charged and dischargedbetween the power supply voltage level and the ground voltage levelaccording to the search data, and there is a room for improvement inreducing the current consumption.

The search operation is performed concurrently on many search and localmatch lines. Therefore, a simultaneous operation current (peak current)is large, which may cause switching noises.

In the article 1, the global and local match lines are charged anddischarged between the levels of the power supply voltage and the groundvoltage. Therefore, signal amplitudes of the local and global matchlines indicating a result of the match-detection are large, and such aproblem arises that there is a limit in reduction of the currentconsumption and the reduction of the time required until settlement ofthe match result. The power supply voltage may be lowered to reduce thesignal amplitude. In this case, however, the operation speed of thetransistor element determines the lower limit of the power supplyvoltage level, to pose the restriction on the increase in operationspeed.

In the construction disclosed in the publication 1, a transistor similarto the CAM cell is used to produce and supply a current of one-bit missstate to the match line. The transistor receiving a reference voltage onits gate is used for charging the match line, to suppress the voltagelevel increase of the match line in the mismatch, at or below thereference voltage. However, such a problem occurs that the match line inthe match state is charged to the power supply voltage level, and thevoltage amplitude thereof becomes large. The publication 1 has notdisclosed a construction for setting the voltage amplitude of the matchline to or below an intermediate voltage level regardless of the matchand mismatch states. Also, no consideration is given to an influencethat is exerted on the match line precharging current by an off-leakagecurrent flowing via the CAM cells in the match state of an entry.

In the construction disclosed in the publication 2, by the chargeredistribution performed by the capacitance division by the match lineand a capacitance element, the precharge voltage level of the match lineis set. Therefore, the capacitance values must be adjusted between thematch line and the capacitance element with high precision, and it isdifficult to charge up precisely the match line in the match state to adesired intermediate voltage level. In the publication 2, the searchline is precharged by short-circuiting the complementary search linesfor reducing the charge/discharge currents of the search lines. However,no consideration is given to reduction of the capacitance of the searchline. Accordingly, the search line is charged from the intermediatevoltage level to the power supply voltage level according to the searchdata, and such problem arises that as the number of entries increases,the load capacitance accordingly increases and the current consumptioncannot be reduced.

In the construction disclosed in the publication 3, the referencevoltage generating circuit and the differential amplifier circuit arearranged for each match line. In this publication 3, however, the matchline is precharged to the power supply voltage level. Therefore, thevoltage amplitude of the match line is large, resulting in a problemthat the fast search operation and the reduced current consumptioncannot be achieved.

In the construction disclosed in the publication 4, the CAM entry isdivided and such a problem arises that the divided entries areprecharged to different voltage levels, and it is difficult to achieve auniform operation speed between the divided entries. In each entry, itis necessary to control the connection of the match line in the dividedentry according to the match/mismatch of the divided entries, and acircuit for such connection requires a large occupation area. Further,the match line of each divided entry has the voltage amplitude of thepower supply voltage level, so that the fast search operation cannot beachieved. When the number of bits of the search data increases andaccordingly the number of bits of the CAM cells of the entry increases,the load of the match line increases to increase the current consumptionfor precharging from the intermediate voltage level to the power supplyvoltage level.

In the construction disclosed in the publication 5, the determinationtiming is set by detecting the voltage level of the dummy match line,and thereby the precharge period of the match line is adjusted. However,no consideration is given to the restriction of the precharge currentvalue. Also, the charging of the match line in the match state is notstopped, and the current consumption in the search operation can bereduced to a limited extent.

In the construction disclosed in the publication 6, the search line andthe bit line are short-circuited to set the voltage amplitude of thesearch line to the intermediate voltage smaller than the power supplyvoltage. Therefore, the bit line must be precharged to the power supplyvoltage level, resulting in a problem that the current consumptioncannot be reduced. The match line is coupled to the buffer (senseamplifier) via the decouple transistor for charging up the match line inthe match state to the intermediate voltage level and for pulling up thesense node to the power supply voltage level. Accordingly, the dischargespeed of the sense node slows in the state of one-bit miss for thesearch data, resulting in a problem that the search operation cannot beperformed fast. The publication 6 discloses another embodiment, in whichredistribution of the accumulated charges of the capacitance elementsets the voltage level of the match line in the match state. This schemeresults in a problem that adjustment of the load capacitances of thecapacitance element and the match line cannot be made easily, similarlyto the publication 2.

SUMMARY OF THE INVENTION

An object of the invention is to provide a content addressable memorythat can reduce current consumption and can achieve a fast searchoperation even when a bit number of search data is large.

Briefly stated, in a content addressable memory according to theinvention, an isolation gate is used to isolate a match line from anamplifier circuit of a match amplifier, to perform a detectionoperation. The match line is precharged to a voltage at or below anintermediate voltage level between the power supply voltage and theground voltage.

According to an embodiment of a first aspect of the invention, a contentaddressable memory includes a plurality of entries each having aplurality of content addressable memory cells; a plurality of matchlines arranged corresponding to the respective entries, each match linecoupled to the content addressable memory cells in a correspondingentry; a search data bus coupled in parallel to the entries andtransferring search data in parallel to the entries; and a plurality ofmatch amplifiers coupled to the match lines, respectively. Each of thematch amplifiers includes a precharge circuit for precharging acorresponding match line to a precharge voltage level equal to or lowerthan an intermediate value between a power supply voltage and a groundvoltage, an amplifier circuit for comparing the voltage on thecorresponding match line with a reference voltage at a voltage levelequal to or lower than the precharge voltage, and an isolation gate forisolating the amplifier circuit from the corresponding match line beforeactivation of the amplifier circuit.

According to an embodiment of a second aspect of the invention, acontent addressable memory includes a plurality of entries each having aplurality of content addressable memory cells; a plurality of matchlines arranged corresponding to the respective entries, each match linecoupled to the content addressable memory cells in a correspondingentry; a search data bus coupled in parallel to the entries andtransferring search data commonly to the entries; and match amplifiersarranged corresponding to the respective match lines and coupled to thecorresponding match lines. Each of the match amplifiers includes anamplifier circuit for comparing a voltage on the corresponding matchline with a reference voltage to produce a signal indicating a result ofthe comparison, a precharge circuit for precharging the correspondingmatch line to a ground voltage level after completion of an amplifyingoperation of the amplifier circuit, and a pull-up current supply circuitfor supplying a current having a restricted current value to thecorresponding match line when the precharge circuit is inactive.

According to an embodiment of a third aspect of the invention, a contentaddressable memory includes a plurality of entries each having aplurality of content addressable memory cells; a plurality of matchlines arranged corresponding to the respective entries, each match linecoupled to the content addressable memory cells in the correspondingentry; a search data bus coupled in parallel to the entries andtransferring search data in parallel to the entries; and a plurality ofmatch amplifiers arranged corresponding to the respective match linesand coupled to the corresponding match lines. Each of the matchamplifiers includes a precharge circuit for precharging thecorresponding match line to a ground voltage level, and a pull-upcurrent supply/determination circuit for supplying a current of arestricted current value to the corresponding match line when theprecharge circuit is inactive, and producing a signal at a voltage levelcorresponding to the voltage level of the corresponding match line. Thecurrent of this restricted current value is smaller than a currentflowing through the corresponding match line in the case of one bit ofthe content addressable memory cells in one entry is made conductive,and is larger than a current flowing through the corresponding matchline when the content addressable memory cells of all the bits arenon-conductive.

According to an embodiment of a fourth aspect of the invention, acontent addressable memory includes a plurality of entries each having aplurality of content addressable memory cells; a plurality of matchlines arranged corresponding to the respective entries, each match linecoupled to the content addressable memory cells in the correspondingentry; a search data bus coupled in parallel to the entries andtransferring search data in parallel to the entries; and a plurality ofmatch amplifiers arranged corresponding to the respective match linesand coupled to the corresponding match lines. Each of the matchamplifiers includes a sensing circuit for producing a signal at avoltage level corresponding to the voltage level of the correspondingmatch line, a latch circuit for latching a signal received from thesensing circuit and corresponding to a voltage level of the match linein a preceding match and detection cycle, and a charge circuit forselectively supplying a current to the corresponding match lineaccording to a latched signal of the latch circuit in a searchoperation.

According to an embodiment of a fifth aspect of the invention, a contentaddressable memory has a plurality of search blocks including aplurality of entries. Each entry includes a plurality of contentaddressable memory cells storing candidate data, and a match linecoupled to the plurality of content addressable memory cells, and beingdriven in a predetermined voltage level direction by the correspondingcontent addressable memory cells according to a result of matchsearching with respect to search data. Each search block furtherincludes a search data bus coupled commonly to the plurality of entriesand transferring the search data. The content addressable memoryaccording to the embodiment of the fifth aspect further includes aplurality of search data input circuits arranged corresponding to thesearch blocks, respectively, and each supplying the search data to thesearch data bus of the corresponding search block, and an activationcontrol circuit for successively activating the plurality of searchblocks and the plurality of search data input circuits according to aclock signal.

According to an embodiment of a sixth aspect of the invention, a contentaddressable memory includes a plurality of entries each having aplurality of content addressable memory cells; a plurality of matchlines arranged corresponding to the respective entries, each match linecoupled to the content addressable memory cells in the correspondingentry; a search data bus coupled in parallel to the entries andtransferring search data in parallel to the entries; and a plurality ofmatch amplifiers arranged corresponding to the plurality of match lines,respectively. Each of the match amplifiers includes a precharge circuitfor precharging a reference voltage node and a corresponding match line,and an amplifier circuit for comparing the voltages on first and secondnodes, and producing a signal indicative of a result of the comparison.The precharge voltage is at a level equal to or lower than anintermediate voltage between a power supply voltage and a groundvoltage. The first node of the amplifier circuit receives the voltage onthe corresponding match line, and the second node is coupled to thereference voltage node. The match amplifier further includes anisolation gate for isolating the corresponding match line and thereference voltage node from the first and second nodes of the amplifiercircuit before activation of the amplifier circuit, and a capacitanceelement for boosting the first node according to a boost instructingsignal before the activation of the amplifier circuit after isolation ofthe isolation gate.

According to an embodiment of a seventh aspect of the invention, acontent addressable memory includes a plurality of entries each having aplurality of content addressable memory cells; a plurality of matchlines arranged corresponding to the respective entries, each match linecoupled to the content addressable memory cells in the correspondingentry; a search data bus for transferring search data in parallel to theentries; and a plurality of match amplifiers arranged corresponding tothe plurality of match lines, respectively. Each of the match amplifiersincludes a precharge circuit for precharging the corresponding matchline, and an amplifier circuit for comparing voltages on first andsecond nodes, and producing a signal indicative of a result of thecomparison. The precharge voltage is at a level equal to or lower thanan intermediate voltage between a power supply voltage and a groundvoltage. The first node receives the voltage on the corresponding matchline. The second node receives a sense reference voltage. The matchamplifier further includes an isolation gate for confining charges onthe first and second nodes before activation of the amplifier circuit.The sense voltage is produced by changing the voltage at a prechargevoltage level using a capacitance element.

According to an embodiment of an eighth aspect of the invention, acontent addressable memory includes a plurality of entries each having aplurality of content addressable memory cells; a plurality of matchlines arranged corresponding to the respective entries, each match linecoupled to the content addressable memory cells in a correspondingentry; a search data bus for transferring search data in parallel to theentries; and a plurality of match amplifiers coupled to the match lines,respectively. Each of the match amplifiers includes a precharge circuitfor precharging the corresponding match line to a precharge voltagelevel equal to or lower than an intermediate value between a powersupply voltage and a ground voltage, an amplifier circuit for comparingthe voltage on the corresponding match line with a reference voltage atthe same voltage level as the precharge voltage, and a capacitanceelement for supplying accumulated charges to the corresponding matchline in the operation of comparing the search data on the search databus with the stored data of the entry.

According to an embodiment of a ninth aspect of the invention, a contentaddressable memory includes a plurality of entries each having aplurality of content addressable memory cells; a plurality of matchlines arranged corresponding to the respective entries, each match linecoupled to the content addressable memory cells in a correspondingentry; a search data bus for transferring search data commonly to theentries; and a plurality of match amplifiers coupled to the respectivematch lines. Each of the match amplifiers includes a precharge circuitfor precharging the corresponding match line to a ground voltage level,and a pull-up current supply/determination circuit supplying, in anoperation of comparing the search data on the search data bus withstored data of each of the entries, a current of a restricted currentvalue to the corresponding match line, clamping an upper limit value ofthe voltage on the corresponding match line at a predetermined value orlower and for producing a signal corresponding to the voltage level ofthe corresponding match line on an internal node.

The invention implements the content addressable memory that performsfast search/determination operations with low current consumption.

Representative effects achieved by the embodiments of the invention areas follows. Since the precharge voltage level of the match line is setto the intermediate voltage level or lower, the charge/dischargecurrents of the match line can be reduced. Also, a signal amplitude ofthe match line is made small, to achieve a fast search operation. In thesearch operation, the amplifier circuit of the match amplifier isactivated while isolating the match line from the amplifier circuit.Thus, full swing of the match line is not required in the operation ofthe amplifier circuit. Thus, the precharge or pull-up of the match linefrom the intermediate voltage level can be performed, so that the signalamplitude of the match line can be reduced, and the current consumptioncan be reduced. When using a cross-coupled type latch amplifier for theamplifier circuit, the driving capacitance of the amplifier circuit isreduced, and the fast amplifying operation can be achieved.

By supplying the current of the restricted current value to the matchline, the match line precharged to the ground voltage level can becharged to the predetermined voltage level according to the comparisonwith the search data, and rising of the voltage level can be suppressedso that the fast search operation and the low current consumption can beachieved. The voltage amplitude of the match line in the search mismatchstate (miss state) in which the charge and discharge of the match lineare performed, can be made smaller than the power supply voltage, andthe current consumption can be reduced.

The restricted current value is smaller than the current flowing fromthe match line through the entry in one-bit miss state, and is largerthan the match line current flowing through the entry in the all-bitnon-conductive state. Thereby, the voltage rising of the match line canbe suppressed even when one-bit mismatch occurs. It is possible tocompensate for voltage lowering due to a leakage current of the matchline in the match state. Thereby, the voltage amplitude of the matchline can be small, and the fast search operation can be achieved withlow current consumption.

The entry is divided into the plurality of search blocks, and the searchoperation is successively performed in the search blocks, so that theconstruction can be equivalent to the construction having divided searchlines. Therefore, it is possible to reduce the charging/dischargingcurrents of the search line, and the fast search operation can beperformed. By successively performing the search operation in the searchblocks, the peak current in the search operation can be reduced, andinfluences by switching noises can be suppressed.

When the sense operation at the voltage level of the match line isperformed by confining the charges, the capacitance element is used tochange the match line voltage. The interconnection capacitance connectedto the capacitance element is small, and the capacitance element canhave a smaller size than that in a construction that uses thecapacitance element for changing the voltage level of the whole matchline, and an occupation area of the capacitance element can be small. Asmall amount of charges can change the voltage significantly. A loadcapacitance of the node that fully swings in the amplifying operation issmall, so that the power consumption can be small.

In the case of producing the sense reference voltage by changing theprecharge voltage using the capacitance element, the precharge voltagelevel is changed using a charge pump operation (capacitive couplingaction) of the capacitance element. In the case of changing the sensereference voltage for the production after confining the charges, theoccupation area of the capacitance element can be small. In the case ofchanging the sense reference voltage by the capacitance element for theproduction before confining the charges, the match amplifier does notrequire a capacitance element for capacitance balance of the sense nodesso that the occupation area can be small. Even when the amplitude of thesense nodes (first and second nodes) of the amplifier circuit fullyswings, the sense nodes have no capacitance element so that the currentconsumption required for charging/discharging the sense nodes can besmall.

In this search operation, the capacitance element supplies theaccumulated charges to the match line to pull up the match line. Thiscan prevent flow of a through current from the power supply node to theground node in the search operation, and can reduce the currentconsumption. The accumulated charges of the capacitance element are usedto pull up the potential of the match line so that the pull-up potentialcan be low, and the increase in voltage amplitude of the match line canbe suppressed.

The current of the restricted value is supplied to the match line, andthe upper limit value of the potential of the match line is clamped sothat the voltage amplitude of the match line can be restricted. Sincethe signal at the voltage level corresponding to the match linepotential is produced, no differential amplifier circuit is required,and the current consumption of the match amplifier can be reduced.Through the clamp function, the internal node of the pull-up currentsupply/determination circuit is isolated from the match line, so thatthe load of the internal node can be reduced, and the voltage level ofthe internal node can be raised fast to the power supply voltage level.Thus, the current consumption can be reduced, and the fastsearch/determination operation can be achieved.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a whole construction of a content addressablememory according to a first embodiment of the invention.

FIG. 2 shows an example of a construction of a unit cell shown in FIG.1.

FIG. 3 shows another construction of the unit cell shown in FIG. 1.

FIG. 4 schematically shows a main portion of the content addressablememory according to the first embodiment of the invention.

FIG. 5 is a timing chart representing an operation of the contentaddressable memory shown in FIG. 4.

FIG. 6 shows an example of a specific construction of a match amplifiershown in FIG. 4.

FIG. 7 schematically shows a construction of a control circuit shown inFIG. 1.

FIG. 8 schematically shows a main portion of a content addressablememory according to a second embodiment of the invention.

FIG. 9 is a timing chart representing an operation of the contentaddressable memory shown in FIG. 8.

FIG. 10 schematically shows a construction of a modification of thecontent addressable memory according to the second embodiment of theinvention.

FIG. 11 schematically shows a whole construction of a contentaddressable memory according to a third embodiment of the invention.

FIG. 12 schematically shows a main portion of the content addressablememory according to the third embodiment of the invention.

FIG. 13 is a timing chart representing an operation of the contentaddressable memory shown in FIG. 12.

FIG. 14 schematically shows a main portion of a content addressablememory of a modification of the third embodiment of the invention.

FIG. 15 schematically shows a main portion of a content addressablememory according to a second modification of the third embodiment of theinvention.

FIG. 16 schematically shows a main portion of a content addressablememory according to a fourth embodiment of the invention.

FIG. 17 is a timing chart representing an operation of the contentaddressable memory shown in FIG. 16.

FIG. 18 schematically shows a construction of a control circuit used ina fourth embodiment of the invention.

FIG. 19 schematically shows a main portion of a content addressablememory according to a fifth embodiment of the invention.

FIG. 20 is a timing chart representing an operation of the contentaddressable memory shown in FIG. 19.

FIG. 21 schematically shows a main portion of a content addressablememory according to a sixth embodiment of the invention.

FIG. 22 is a timing chart representing an operation of the contentaddressable memory shown in FIG. 21.

FIG. 23 schematically shows a construction of a modification of a biasvoltage generating portion of the content addressable memory accordingto the sixth embodiment of the invention.

FIG. 24 schematically shows a construction of a control circuit of thecontent addressable memory according to the sixth embodiment of theinvention.

FIG. 25 schematically shows a main portion of a content addressablememory according to a seventh embodiment of the invention.

FIG. 26 is a timing chart representing an operation of the contentaddressable memory shown in FIG. 25.

FIG. 27 schematically shows a main portion of a content addressablememory according to an eighth embodiment of the invention.

FIG. 28 is a timing chart representing an operation of the contentaddressable memory shown in FIG. 27.

FIG. 29 schematically shows a construction of a control signalgenerating portion of the content addressable memory according to theeighth embodiment of the invention.

FIG. 30 schematically shows a main portion of a content addressablememory according to a ninth embodiment of the invention.

FIG. 31 schematically shows a main portion of a content addressablememory according to a tenth embodiment of the invention.

FIG. 32 shows an example of a construction of a buffer shown in FIG. 31.

FIG. 33 schematically shows a main portion of a content addressablememory according to an eleventh embodiment of the invention.

FIG. 34 shows an example of a construction of a current convertercircuit shown in FIG. 33.

FIG. 35 shows another construction of the current converter circuitshown in FIG. 33.

FIG. 36 shows an example of a construction of a buffer shown in FIG. 33.

FIG. 37 schematically shows a main portion of a content addressablememory of a modification of the eleventh embodiment of the invention.

FIG. 38 schematically shows a main portion of a content addressablememory according to a twelfth embodiment of the invention.

FIG. 39 illustrates, in a table form, operation logic of a chargecircuit shown in FIG. 34.

FIG. 40 is a timing chart representing an operation of the contentaddressable memory shown in FIG. 38.

FIG. 41 illustrates, in a table form, charge consumption per searchcycle of the content addressable memory shown in FIG. 38.

FIG. 42 shows a main portion of a modification of the twelfth embodimentof the invention.

FIG. 43 is a timing chart representing an operation of a match amplifiershown in FIG. 42.

FIG. 44 schematically shows a main portion of a content addressablememory according to a thirteenth embodiment of the invention.

FIG. 45 schematically shows a main portion of a content addressablememory according to a fourteenth embodiment of the invention.

FIG. 46 is a timing chart representing an operation of the contentaddressable memory shown in FIG. 45.

FIG. 47 schematically shows a whole construction of a contentaddressable memory according to a fifteenth embodiment of the invention.

FIG. 48 is a timing chart representing an operation of a delay controlcircuit of the content addressable memory shown in FIG. 47.

FIG. 49 is a timing chart representing an operation of the contentaddressable memory shown in FIG. 47.

FIG. 50 shows an example of a construction of a priority encoder shownin FIG. 47.

FIG. 51 shows a main portion of a CAM according to a sixteenthembodiment of the invention.

FIG. 52 is a timing chart representing an operation of a match amplifiershown in FIG. 51.

FIG. 53 schematically shows a whole construction of the CAM according tothe sixteenth embodiment of the invention.

FIG. 54 schematically shows a construction of a portion for generatingcontrol signals shown in FIG. 51.

FIG. 55 schematically shows a main portion of a CAM according to aseventeenth embodiment of the invention.

FIG. 56 is a timing chart representing an operation of a match amplifiershown in FIG. 55.

FIG. 57 schematically shows a construction of a portion for generating acontrol signal shown in FIG. 55.

FIG. 58 schematically shows a main portion of a CAM according to aneighteenth embodiment of the invention.

FIG. 59 is a timing chart representing an operation of a match amplifiershown in FIG. 58.

FIG. 60 schematically shows a construction of a portion for generatingcontrol signals shown in FIG. 58.

FIG. 61 schematically shows a main portion of a CAM according to anineteenth embodiment of the invention.

FIG. 62 is a timing chart representing an operation of a match amplifiershown in FIG. 61.

FIG. 63 shows a main portion of a CAM according to a twentiethembodiment of the invention.

FIG. 64 schematically shows a main portion of a CAM according to atwenty-first embodiment of the invention.

FIG. 65 is a timing chart representing an operation of a match amplifiershown in FIG. 64.

FIG. 66 schematically shows a construction of a portion generating acontrol signal shown in FIG. 61.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

FIG. 1 schematically shows a whole construction of a content addressablememory according to a first embodiment of the invention. In FIG. 1, thecontent addressable memory includes a memory cell array 1 having unitcells UC arranged in rows and columns. Memory cell array 1 is dividedinto a plurality of entries ERY. A match line ML is arranged for eachentry ERY, and each match line ML is coupled to unit cells UC in acorresponding entry in parallel. Each of search line pairs SLP fortransmitting search data is arranged commonly to entries ERY in memorycell array 1. Unit cell UC is arranged corresponding each crossingbetween search line pairs SLP and match lines ML. Unit cell UC has aconstruction as described later and has a function of storing andsearching data. The unit cell can have various constructions.Accordingly, the term “unit cell” is used instead of “contentaddressable memory cell” and “CAM cell” in the following description.

The content addressable memory further includes a match determiningcircuit 2 arranged corresponding to entries ERY for determiningmatch/mismatch of the search data with stored data of each entry. Matchdetermining circuit 2 includes match amplifiers 10 coupled to therespective match lines. The content addressable memory further includesan intermediate voltage generating circuit 6 for supplying anintermediate voltage VML and a comparison reference voltage VREF tomatch amplifiers 10 in match determining circuit 2, a search data inputcircuit 4 that receives externally supplied search data SD and transmitsthe search data SD to search line pairs SLP of memory cell array 1, anda control circuit 8 responsive to a clock signal CLK for controllingoperations of match determining circuit 2 and search data input circuit4 according to an externally supplied command CMD instructing anoperation mode.

Intermediate voltage generating circuit 6 produces an intermediatevoltage at a voltage level of half a power supply voltage VDD or lowerfrom power supply voltage VDD. Intermediate voltage VML and comparisonreference voltage VREF may be at the same voltage level, or intermediatevoltage VML may be higher than comparison reference voltage VREF.Intermediate voltage VML is used as a precharge voltage for prechargingeach match line ML via match amplifier 10. An amplitude of match line MLis set to or lower than half the power supply voltage VDD/2, whereby thecurrent consumption is reduced, and a search operation speed isincreased.

FIG. 2 shows an example of a construction of unit cell UC shown inFIG. 1. In FIG. 2, unit cell UC includes an SRAM cell SMC storing onebit of data, N-channel MOS transistors (insulated gate field effecttransistors) TR1 and TR2 connected in series between match line ML and aground node, and N-channel MOS transistors TR3 and TR4 arranged inseries between match line ML and the ground node. MOS transistors TR1and TR3 have gates coupled to search lines SL and /SL, respectively. MOStransistors TR2 and TR4 have gates coupled to internal storage nodes /Dand D of SRAM cell SMC, respectively. These internal storage nodes D and/D store data bits complementary to each other. When SRAM cell SMCstores “1”, internal storage nodes D and /D are at H and L levels,respectively. In this state, therefore, MOS transistor TR2 isconductive, and MOS transistor TR4 is non-conductive. When SRAM cell SMCstores data “0”, the opposite state is attained.

Search lines SL and /SL form search line pair SLP shown in FIG. 1, andtransmit complementary data in a search operation. For unit cell UCshown in FIG. 2, there are provided a word line and a bit line pair forwriting and reading data into/from SRAM cell SMC, although FIG. 2 doesnot show the word line and bit line pair.

In the search operation, when search data “1” is supplied while SRAMcell SMC has stored “1” (internal storage node D is at the H level),search line SL is at the H level, and complementary search line /SL isat the L level. Therefore, MOS transistors TR2 and TR3 are off, andmatch line ML is kept at the precharge voltage level. Conversely, whenthe search data of “0” is transferred to search line SL while thepotential of internal storage node D of SRAM cell SMC is at the H level,search line SL attains the L level, and complementary search line /SLattains the H level. In this case, MOS transistors TR3 and TR4 are bothmade conductive, and match line ML is discharged from the prechargevoltage level to the ground voltage level.

Therefore, with the construction using unit cell UC shown in FIG. 2,binary determination can be made on match/mismatch between the searchdata and the storage data of the entry. Unit cells UC in the entry arecoupled in parallel to corresponding match line ML. When all unit cellsUC in entry ERY are in the match state, match line ML holds theprecharge voltage level. When the entry includes at least one bit ofunit cell in the mismatch state (miss state), match line ML isdischarged via this unit cell in the mismatch state, and the potentialof match line ML lowers from the precharge voltage level. Therefore, byamplifying the potential level of match line ML by match amplifier 10 inmatch determining circuit 2, it is possible to perform the binarydetermination on match/mismatch between the search data and the storeddata of each entry.

FIG. 3 shows another construction of unit cell UC shown in FIG. 1. InFIG. 3, unit cell UC differs from unit cell UC shown in FIG. 2 in thefollowing construction. As the data storage element, first and secondcells MC1 and MC2 each having a logical value of storage data setseparately and individually are employed in place of SRAM cell SMC.Storage nodes ND1 and ND2 of first and second cells MC1 and MC2 arecoupled to the gates of MOS transistors TR2 and TR4, respectively. Eachof first and second cells MC1 and MC2 is implemented using a DRAM memorycell as described in the article 1. Charges accumulated in a capacitorare used to store data. Other constructions of unit cell UC shown inFIG. 3 are the same as those of unit cell UC shown in FIG. 2.Corresponding portions are allotted with the same reference numbers, anddescription thereof is not repeated.

In FIG. 3, the word and bit lines for writing/reading data are arrangedfor each of first and second cells MC1 and MC2. However, FIG. 3 does notshow the word and bit lines for writing/reading data for the sake ofsimplicity, similarly to FIG. 2.

In unit cell UC shown in FIG. 3, when first and second cells MC1 and MC2store the complementary data, unit cell UC performs the search operationaccording to the same logic as unit cell UC in FIG. 2. Thus, match lineML is discharged when mismatch (miss) occurs. When match (hit) occurs,match line ML is kept at the precharge voltage.

When both first and second cells MC1 and MC2 store data “0” at the Llevel, both MOS transistors TR2 and TR4 are off. In this state,therefore, match line ML is not discharged regardless of the logicalvalue of search data, and match line ML keeps the precharged state. Thiscan implement the “don't care” state.

When both first and second cells MC1 and MC2 have stored data “1” at theH level, both MOS transistors TR2 and TR4 are on. In this case, matchline ML is discharged regardless of the value of search data. In thisstate, the stored data of the entry is invalid (i.e., normally in themismatch state) regardless of the search data. Unit cell UC of theconstruction shown in FIG. 3 can make ternary determination of match,mismatch and don't care.

By using unit cell UC shown in either of FIGS. 2 and 3 for the CAM(Content Addressable Memory) cell, match line NIL is discharged via apath of MOS transistors TR1 and TR2 or a path of MOS transistors TR3 andTR4 in the miss state.

FIG. 4 shows a specific construction of match amplifier 10 in thecontent addressable memory according to the first embodiment of theinvention. In FIG. 4, in memory cell array 1, there are (n+1) entriesERY0-ERYn. Match lines ML[0]-ML[m] are arranged corresponding to entriesERY0-ERYn, respectively. Each of entries ERY0-ERYn includes a pluralityof unit cells UC. A memory cell (CAM cell) CC arranged for data storagein each unit cell UC may be formed of SRAM cell SMC shown in FIG. 2 ormemory cells MC1 and MC2 shown in FIG. 3. In the following description,CAM cell CC refers to either of the unit cell performing the binarydetermination and the unit cell performing the ternary determination.

Match amplifiers 10 are arranged corresponding to entries ERY0-ERYn,respectively. FIG. 4 representatively shows a construction of matchamplifier 10 arranged corresponding to entry ERY0. Match amplifier 10includes a differential amplifier circuit 12 for comparing a voltage oncorresponding match line ML (ML[0]) with a reference voltage VREF, alatch 16 for latching an output signal of differential amplifier circuit12 according to a latch instructing signal LAT and producing a searchresult indicating signal ML_OUT (ML_OUT[0]), and a precharge transistor14 for transmitting precharge voltage VML to corresponding match line ML(ML[0]) in response to activation of a precharge instructing signalPRE_n.

Differential amplifier circuit 12 includes a differential amplifier 12 ahaving a positive input (+) coupled to match line ML and a negativeinput (−) receiving reference voltage VREF, and an amplifier activationtransistor 12 b for activating differential amplifier 12 a in responseto a match amplifier activating signal MAE.

Precharge voltage VML is at a level of half power supply voltage VDD,and reference voltage VREF is lower than precharge voltage VIAL(0<VREF<VML≦VDD/2).

Search lines SL and /SL are precharged to the ground voltage levelduring standby, and are selectively driven to the power supply voltagelevel according to the search data in the search operation.

FIG. 5 is a timing chart representing an operation of the contentaddressable memory shown in FIG. 4. FIG. 5 shows operating waveformsrelating to one entry. Referring to FIG. 5, a search operation of thecontent addressable memory shown in FIG. 4 will now be described.

In a standby state before a time T1, search lines SL and /SL are at thelevel of ground voltage GND, and match line ML is also at the level ofground voltage GND.

The search operation starts at time T1. In response to start of thesearch operation at time T1, precharge instructing signal PRE_n attainsthe L level so that precharge transistor 14 is turned on for each entry,to precharge each of match lines ML (ML[0]-ML[n]) to the intermediatevoltage level, the level of precharge voltage VML. During a periodbetween times T1 and T2, both search lines SL and /SL are kept at theground voltage level.

At time T2, the precharge operation completes, search lines SL and /SLare activated and a comparison is made between the stored data and thesearch data. In this activation/comparison cycle, precharge instructingsignal PRE_n is at the H level to maintain precharge transistor 14 off.Search lines SL and /SL receive the search data, and are each driven tothe voltage level corresponding to the bit value of the search data.Thereby, in entries ERY0-ERYn, the search operation is performed inparallel. According to match/mismatch between the stored data of CAMcell CC and the search data, each unit cell UC selectively dischargesthe corresponding match line. As shown in FIG. 5, when at least one bitof the unit cell in the entry is in the mismatch state, match line ML ofthis miss-state entry is discharged via a path of MOS transistors TR1and TR2 or transistors TR3 and TR4 in unit cell UC in the mismatchstate, and the precharge voltage level of match line ML lowers.

At a time T3, when the voltage level of match line ML is sufficientlydeveloped, match amplifier activating signal MAE is made active.Thereby, a determination cycle starts overlapping with a data comparisoncycle, and differential amplifier circuit 12 performs the differentialamplification. Thus, differential amplifier circuit 12 produces a signalcorresponding to a difference between reference voltage VREF and thepotential of corresponding match line ML. At time T3, latch instructingsignal LAT attains the H level, and a match amplifier output cyclestarts in parallel. In this cycle, latch 16 enters the through state andlatches the output signal of differential amplifier circuit 12 andtransfers the latched data to an output node. In FIG. 5, thecorresponding entry is in the mismatch state, and search resultindicating signal ML_OUT (one of ML_OUT[0]-ML_OUT[n]) changes to theground voltage level.

When the output signal of latch 16 is made definite, the determinationcycle ends at a time T4, and a cycle for outputting the search resultstarts. Match amplifier activating signal MAE is made inactive, andlatch instructing signal LAT is driven to the L level. Search lines SLand /SL are precharged to the ground voltage level again. When latchinstructing signal LAT attains the L level, latch 16 enters the latchstate to keep its output signal ML_OUT at the L level indicative of themismatch state.

One search cycle is formed of the period between times T1 and T5. Insynchronization with clock signal CLK, the content addressable memorysuccessively execute a series of searching operations including thematch line precharging, search line activation, data comparison,determination and output of the determination result.

At time T5, a next search cycle starts, and precharge instructing signalPRE_n attains the L level so that match line ML is precharged to theintermediate voltage level of precharge voltage VML again. The voltagelevel of the match line in the match state slightly lowers due to anoff-leakage current of the unit cell in the corresponding entry.However, the amount of voltage lowering over consecutive cycles in thematch entry is small, and the match line in the match statesubstantially is held at the precharge voltage level. The “off-leakagecurrent” represents a current flowing through the paths of transistorsTR1 and TR2 or transistors TR3 and TR4 in the off state.

At a time T6, the voltage levels of search lines SL and /SL are setagain according to the search data, and the comparison between thestored data and the search data is performed in each entry.

At a time T7, match amplifier activating signal MAE is made activeagain, and latch instructing signal LAT is driven to the H level. In thematch state where the search data matches the stored data of the entry,all unit cells UC in the entry are non-conductive, and a dischargingpath is not present for the corresponding match line, and therefore,match line ML is held substantially at the level of precharge voltageVML. Accordingly, latch 16 produces and latches signal ML_OUT at the Hlevel indicative of the match state.

At a time T8, match amplifier activating signal MAE is made inactive,and latch 16 enters the latch state. Subsequently, the search cycle isrepeated a number of times corresponding to the number of search data.

In the construction shown in FIG. 4, as described above, match line MLchanges between ground voltage GND and precharge voltage VML, and thepotential of match line ML is compared with reference voltage VREF,Match amplifier 10 converts a signal of a small amplitude appearing onmatch line ML into a signal of a full amplitude of the power supplyvoltage level, to produce search result indicating signal ML_OUT.Therefore, the voltage amplitude of the match line can be small in thesearch cycle, and the charging/discharging currents of match line ML canbe small. Match lines ML in the mismatch state are much larger in numberthan match line ML in the match state, and the amplitude restriction ofthe match lines can significantly reduce the charging/dischargingcurrents of the match lines.

Differential amplifier circuit 12 is used for comparing referencevoltage VREF with the voltage on match line ML, so that the fast senseoperation can be performed. In the one-bit miss state, i.e., when onebit (unit cell) out of unit cells is in the mismatch state in entry ERY,the current of the corresponding match line is pulled out slowly.However, precharge voltage VML of match line ML is equal to or lowerthan intermediate voltage VDD/2, and the voltage level of thecorresponding match line can rapidly attain the level lower thanreference voltage VREF even in the one-bit miss state so that thepotential level of the match line can be determined at a faster timing.Thereby, the content addressable memory can be implanted to achieve thefast search with low current consumption. Here, the term “mismatchstate” is used to refer to the state where the search data mismatchesthe stored data in an entry as a whole, and the term “miss state” isused to refer to the state where storage data bit a unit cell mismatchesthe corresponding search data bit, or the mismatch on a unit cell basis.

FIG. 6 shows an example of specific constructions of differentialamplifier circuit 12 and latch 16 shown in FIG. 4. In FIG. 6,differential amplifier 12 a of differential amplifier circuit 12includes an N-channel MOS transistor NQ1 having a gate coupled to matchline ML, an N-channel MOS transistor NQ2 receiving reference voltageVREF on its gate, and P-channel MOS transistors PQ1 and PQ2 supplyingcurrents to MOS transistors NQ1 and NQ2, respectively. Sources of MOStransistors NQ1 and NQ2 are commonly coupled to a drain of activationtransistor 12 b. MOS transistors PQ1 and PQ2 form a current mirrorstage. MOS transistor PQ1 has a gate and a drain interconnected witheach other, and serves as a master of the current mirror stage.

Latch 16 includes an inverter IV1 inverting latch instructing signalLAT, a tristate inverter buffer BV that selectively becomes activeaccording to latch instructing signal LAT and the output signal ofinverter IV1, and inverters IV2 and IV3 coupled to the output oftristate inverter buffer BV. Inverter IV2 inverts the output signal oftristate inverter buffer BY to produce search result indicating signalML_OUT. Inverter IV3 inverts the output signal of inverter IV2, andprovides the inverted signal to an input of inverter IV2. Inverters IV1and IV2 form a so-called inverter latch.

In differential amplifier circuit 12, currents of the same magnitudeflow through MOS transistors PQ1 and PQ2, respectively. When the voltagelevel of match line ML is higher than reference voltage VREF, MOStransistor NQ1 causes a current flow larger than MOS transistor NQ2does. MOS transistor PQ2 supplies a mirror current of the currentflowing through MOS transistor PQ1 to MOS transistor NQ2. In this case,therefore, MOS transistor NQ2 cannot wholly discharge the currentsupplied from MOS transistor PQ2, and differential amplifier 12 agenerates an output signal at the H level.

When the voltage level of match line ML is lower than reference voltageVREF, MOS transistor NQ2 has a larger conductance than MOS transistorNQ1. In this state, the current supplied from MOS transistor PQ2 isentirely discharged via MOS transistor NQ2 and activation transistor 12b, and differential amplifier 12 a generates the output signal at the Llevel.

In latch 16, when latch instructing signal LAT is at the L level,tristate inverter buffer BV is in the output high-impedance state, andoutput signal ML_OUT thereof does not change. When latch instructingsignal LAT attains the H level, tristate inverter buffer BV operates asan inverter to amplify further the output signal of differentialamplifier circuit 12. Inverters IV2 and IV3 latch and output the signalthus amplified.

Therefore, precharge transistor 14 precharges match line ML to the levelof intermediate voltage VML, and the sense operation can be performedfast immediately when the difference between the voltage on match lineML and reference voltage VREF attains a value sensible by differentialamplifier 12 a, even in the case where the signal amplitude of matchline ML is small.

FIG. 7 shows an example of a construction of control circuit 8 shown inFIG. 1. In FIG. 7, control circuit 8 includes a command decoder 20 thatdecodes command CMD supplied in synchronization with clock signal CLK,and a precharge activating circuit 22 that drives and maintainsprecharge instructing signal PRE_n to and at the L level for apredetermined period according to a search operation instruction ENsupplied from command decoder 20.

Control circuit 8 includes a search line drive activating circuit 24changes its output logical level using the change of clock signal CLK tothe L level as a trigger when search operation instruction EN is active,a delay circuit 26 that delays search operation instruction EN by oneclock cycle of clock signal CLK, and a match amplifier activatingcircuit 28 that produces match amplifier activating signal MAE and latchinstructing signal LAT according to the output signal of delay circuit26 and clock signal CLK.

Command decoder 20 decodes command CMD in synchronization with therising of clock signal CLK, and activates search operation instructionEN when decoded command CMD instructs the search operation. Prechargeactivating circuit 22 is formed of, e.g., a gate circuit receiving clocksignal CLK and search operation instruction EN, and supplies prechargeinstructing signal PRE_n at the L level when clock signal CLK is at theH level and search operation instruction EN is at the H level.

Search line drive activating circuit 24 is formed of, e.g., a T-typeflip-flop, and activates its output signal (i.e., a search line driveenable signal SLEN) using the falling of clock signal CLK as a triggerwhen search operation instruction EN is active. Search line driveactivating circuit 24 supplies search line activation instructing signalSLEN to search data input circuit 4. Search data input circuit 4 takesin search data SD that is supplied when search operation instruction ENis active, and drives the search line according to taken search data SDwhen search line drive activation instructing signal SLEN is active.When search line drive activation instructing signal SLEN is inactive,search data input circuit 4 keeps both search lines SL and /SL at the Llevel.

Match amplifier activating circuit 28 is formed of, e.g., a gate circuitreceiving the output signal of delay circuit 26 and clock signal CLK,and holds match amplifier activating signal MAE and latch instructingsignal LAT at the H level when clock signal CLK and the output signal ofdelay circuit 26 are both at the H level.

By using control circuit 8 shown by way of example in FIG. 7, the matchline can be precharged at the start of the search operation, andsubsequently the search line can be driven according to the search dataafter completion of the precharging. Further, in a next cycle of clocksignal CLK during driving of the search line, match amplifier activatingsignal MAE and latch instructing signal LAT can be driven to the H levelfor half the clock cycle period. Thereby, the control circuit canachieve the timings of the timing control signals as shown in FIG. 5.

For the circuit for generating intermediate voltage VML to betransmitted to the match line, any circuit that can produce a voltage ata level not higher than half the power supply voltage VDD and not lowerthan reference voltage VREF, can be used. Intermediate voltagegenerating circuits having various constructions can be used for theprecharge voltage generating circuit.

According to the first embodiment of the invention, the prechargevoltage level of the match line is set to the level of half the powersupply voltage or lower, and the match line voltage is compared with thereference voltage lower than the precharge voltage, to produce thesignal indicative of the search result. Therefore, the charge/dischargecurrents of the match line can be small, and the signal amplitude of thematch line can be small. Thereby, the content addressable memory cansense the match line voltage at high speed, and can achieve the fastoperation with low current consumption.

Second Embodiment

FIG. 8 shows a main portion of a content addressable memory according toa second embodiment of the invention. The content addressable memoryshown in FIG. 8 differs from that shown in FIG. 4 in internalconstruction of latch amplifier 10. Specifically, an isolation gatecircuit 30 is arranged at a preceding stage of differential amplifiercircuit 12. Isolation gate circuit 30 includes isolation gates (chargeconfining gates) 30 a and 30 b that are selectively turned off accordingto an isolation instructing signal MLI. Isolation gate 30 a selectivelyisolates match line ML from a positive input (+) of differentialamplifier 12 a. Isolation gate 30 b shuts off transmission of referencevoltage VREF to a negative input (−) of differential amplifier 12 a.Other internal constructions shown in FIG. 8 are the same as those ofthe content addressable memory shown in FIG. 4. Corresponding portionsare allotted the same reference numerals, and description thereof is notrepeated.

FIG. 9 is a timing chart representing the search operation of thecontent addressable memory shown in FIG. 8, Referring to FIG. 9, thesearch operation of the content addressable memory shown in FIG. 8 willnow be described.

The search cycle starts at time T1. At time T1, precharge instructingsignal PRE_n attains the L level in synchronization with the rising ofclock signal CLK, and each match line ML is precharged to theintermediate voltage level of precharge voltage VML.

Subsequently at time T2, search lines SL and /SL are driven from theground voltage level to the voltage level corresponding to the searchdata according to the falling of clock signal CLK. When there ismismatch between the search data and the stored data in the entrycorresponding to match line ML, the voltage level of match line MLlowers below reference voltage VREF.

At time T3, isolation instructing signal MLI is driven to the L level,and isolation gates 30 a and 30 b are turned off. Concurrently, matchamplifier activating signal MAE and latch instructing signal LAT aredriven to the H level. Thereby, differential amplifier circuit 12 isactivated to perform the differential amplification. Also, latch 16enters the through state to produce a signal according to the outputsignal of differential amplifier circuit 12.

At time T3, differential amplifier circuit 12 has already received onits positive and negative inputs the change in voltage level of matchline ML and reference voltage VREF, respectively. Differential amplifiercircuit 12 makes comparison (determination) between the transmittedmatch line voltage and the reference voltage. Isolation gate circuit 30has already isolated match line ML from differential amplifier circuit12. In this state, therefore, it is not necessary to drive search linesSL and /SL according to the search data, and search lines SL and /SL aredriven to the ground voltage level again.

Differential amplifier circuit 12 differentially amplifies the voltageson its positive and negative inputs according to the voltage levelscorresponding to the charges confined by isolation gate circuit 30.

In the amplifying operation of differential amplifier circuit 12, matchline ML is isolated from differential amplifier circuit 12, and searchlines SL and /SL are set to the ground voltage level. Therefore, matchline ML has the charge/discharge operation stopped, and is notdischarged to the level of ground voltage GND.

At time T4, the data determining operation is completed, and latchinstructing signal LAT attains the L level. Thereby, latch 16 enters thelatch state, and signal ML_OUT indicative of a result of the comparisonenters a definite state. FIG. 9 shows the miss state, and signal ML_OUTat the L level is produced.

At time T5, the search cycle restarts, and match line ML is precharged.In this case, match line ML is at a voltage level higher than groundvoltage GND, and is driven fast to the level of precharge voltage VML.Similarly to the previous search cycle, the search line is drivenaccording to the search data, isolation gates 30 a and 30 b confine thecharges and differential amplifier circuit 12 performs theamplification.

In the search cycle starting from time T5 in FIG. 9, the search datamatches the stored data, and signal ML_OUT at the H level indicative ofthe match state is produced.

By using isolation gates 30 a and 30 b of isolation gate circuit 30,match line ML is isolated from differential amplifier circuit 12 duringthe sense operation (operation of the match amplifier). Thereby, thevoltage amplitude of match line ML can be further reduced, and thecurrent consumption can be further reduced. Also, it is possible tocomplete the precharge operation at a faster timing.

When the sense (sensing operation) is performed according to the chargeconfining scheme using isolation gate circuit 30, the differentialamplifier circuit of the current mirror type shown in FIG. 6 may be usedas differential amplifier circuit 12. However, when the sensingoperation is to be performed according to the charge confining scheme, aso-called cross-coupled, latch type sense amplifier may be used for thedifferential amplifier circuit, whereby the search operation can beperformed fast and efficiently.

FIG. 10 shows another construction of differential amplifier circuit 12shown in FIG. 8. FIG. 10 shows constructions of the match amplifierscorresponding to match lines ML[i] and ML[i+1]. Differential amplifiercircuit 12 and latch 16 for each match line have the sameconfigurations. In FIG. 10, components in the match amplifier arrangedfor match line ML[i] representatively are allotted reference numerals.

In FIG. 10, differential amplifier 12 a includes P-channel MOStransistors PQ3 and PQ4 having their gates and drains cross-coupled,N-channel MOS transistors NQ3 and NQ4 having their gates and drainscross-coupled, and an activating P-channel MOS transistor PQ5. When acomplementary match amplifier activating-signal MAEZ is active,P-channel MOS transistor PQ5 couples the power supply node to sourcenodes of MOS transistors PQ3 and PQ4 when complementary match amplifieractivating signal MAEZ.

The drains of MOS transistors PQ3 and NQ3 as well as the gates of MOStransistors PQ4 and NQ4 are coupled to match line ML[i] via isolationgate 30 a. The gates of MOS transistors PQ3 and NQ3 as well as thedrains of MOS transistors PQ4 and NQ4 commonly receive reference voltageVREF via isolation gate 30 b.

A match amplifier activation transistor 12 b is further employed fordifferential amplifier 12 a. Match amplifier activation transistor 12 bcouples the sources of MOS transistors NQ3 and NQ4 to the ground inresponse to match amplifier activating signal MAE.

Latch 16 includes inverter IV1 receiving latch instructing signal LAT, atristate inverter buffer BV1 coupled to the drain of MOS transistor PQ3and the gate of MOS transistor PQ4 in differential amplifier 12 a, and atristate inverter buffer BV2 coupled to the gate of MOS transistor NQ3and the drain node of MOS transistor NQ4. Tristate inverter buffer BV1is selectively activated in response to the output signal of inverterIV1 and latch instructing signal LAT. Tristate inverter buffer BV2 isselectively activated according to latch instructing signal LAT and theoutput signal of inverter IV1, but its output is in an open state.

Latch 16 further includes inverters IV2 and IV3 forming an inverterlatch. Inverters IV2 and IV3 latches the output signal of tristateinverter buffer BV1 to produce search result indicating signalML_OUT[i].

In latch 16 shown in FIG. 10, tristate inverter buffer BV2 is arrangedto the node receiving reference voltage VREF for the following reason.Sense nodes ND1 and ND2 of differential amplifier 12 a are made the samein load in the sense operation. Since the sense nodes ND1 and ND2 arethe same in load, the cross-coupled sense amplifier forming differentialamplifier 12 a can precisely perform the sense operation.

In differential amplifier circuit 12 shown in FIG. 10, match amplifieractivating signals MAE and MAEZ are inactive during the precharge periodand search line drive period, and MOS transistor PQ4 and match amplifieractivation transistor 12 b are off in these periods. In isolation gatecircuit 30 a, isolation gates 30 a and 30 b are conductive. Even whensense nodes ND1 and ND2 are precharged to the levels of intermediatevoltage VML and reference voltage VREF, respectively, the movement ofthe charges is prevented between the reference voltage line and thecorresponding match line via the internal nodes of differentialamplifier 12 a. Thus, sense node ND1 is at a higher level than referencevoltage VREF, and MOS transistor PQ4 is non-conductive. Also, MOStransistor 12 b is non-conductive. Therefore, even when MOS transistorNQ4 is turned on, the source node potential thereof is at the level ofreference voltage VREF, and MOS transistor NQ4 is kept non-conductive.

MOS transistor PQ3 receives reference voltage VREF on its gate.Therefore, even when MOS transistor PQ3 is turned conductive, thevoltages on its source and drain are equalized when its source noderises to the level of precharge voltage VML, so that MOS transistor PQ3is turned non-conductive. Likewise, even when MOS transistor NQ3 isturned on according to reference voltage VREF, MOS transistor NQ4charges the common source node of MOS transistors NQ3 and NQ4 to thelevel of reference voltage VREF. Therefore, MOS transistor NQ3 has thesame potential at its gate and source, and keeps the non-conductivestate. Therefore, sense nodes ND1 and ND2 are held at the levels ofprecharge voltage VML and reference voltage VREF during the prechargeoperation, respectively.

Then, the search (data comparison) operation is executed. Even when thepotential of match line ML changes according to the search data, and thepotential of sense node ND1 changes, the common source node of MOStransistors PQ3 and PQ4 is kept at the level of precharge voltage VML,and the common source node of MOS transistors NQ3 and NQ4 is kept at thelevel of reference voltage VREF. Thereby, MOS transistors PQ3, PQ4, NQ3and NQ4 keep the non-conductive state. Therefore, sense node ND1 is setto the voltage level corresponding to the potential of match line ML[i].

Isolation instructing signal MLI is set to the L level, and therebyisolation gate circuit 30 is turned non-conductive to confine thecharges. Also, match amplifier activating signal MAE is activated.Accordingly, MOS transistor PQ5 and activation transistor 12 b areturned conductive, and the sense operation (determination operation) isperformed. Of sense node ND1 or ND1, the node at a higher potential ispulled up to the level of power supply voltage VDD by MOS transistor PQ3or PQ4. The sense node at a lower potential is discharged to the levelof the ground voltage by MOS transistor NQ3 or NQ4. Differentialamplifier 12 a is a latch type of differential amplifier circuit. Whilematch amplifier activating signals MAE and MAEZ are active, sense nodesND1 and ND2 latch the signals at the levels of amplified power supplyvoltage VDD and ground voltage.

As shown in FIG. 10, the cross-coupled sense amplifier is used for theamplifier in the match amplifier circuit, and the voltage sensing isperformed according to the charge confining scheme. Thereby, the loadson sense nodes ND1 and ND2 of differential amplifier 12 a can be small,and the amplifying operation can be performed fast. During theamplifying operation of differential amplifier 12 a, isolation gatecircuit 30 is off, and for each of match lines ML (ML[i] and ML[i+1]),the discharging operation can be stopped.

In the second embodiment of the invention, for the control circuit, thecircuit substantially the same construction as the first embodiment canbe employed. Thus, in the construction of control circuit 8 shown inFIG. 7, the search line drive activating circuit activates search lineactivating signal SLEN while clock signal CLK is at the L level.Further, as for isolation instructing signal MLI, isolation instructingsignal MLI is driven to and maintained at the L level according to theoutput signal of delay circuit 26 shown in FIG. 8 during one clock cycleof clock signal CLK.

According to the second embodiment of the invention, as described above,the match amplifier senses the voltage level of the match line accordingto the charge confining scheme. Therefore, the voltage amplitude of thematch line can be further reduced, and the voltage sensing of the matchline can be performed fast.

Third Embodiment

FIG. 11 schematically shows a whole construction a content addressablememory according to a third embodiment of the invention. Similarly tothe first and second embodiments, in the content addressable memoryshown in FIG. 11, memory cell array 1 is divided into a plurality ofentries ERY . Match line ML is arranged for each entry ERY , and eachsearch line pair SLP (search lines SL and /SL) is arranged for allentries ERY . The plurality of search line pairs constitute a searchdata bus. Match determining circuit 2 includes match amplifiers 40provided corresponding to respective entries ERY . Match amplifier 40has a pull-up function of supplying a pulling-up current to acorresponding match line in the data comparing operation. A bias voltagegenerating circuit 45 is arranged for controlling the pull-up currentsupply in match amplifier 40. Match amplifier 40 supplies a pull-upcurrent of a restricted current value to corresponding match line MLaccording to a bias voltage BIAS_P provided from bias voltage generatingcircuit 45.

An intermediate voltage generating circuit 42 produces precharge voltageVML, as a comparison determination reference voltage, to match amplifier40 in match determining circuit 2. Therefore, reference voltage VREF isnot used. Precharge voltage VML is used as a precharge voltage of thematch line, and is also used in the operation of determining the voltagelevel of the match line so that the occupation area and currentconsumption of intermediate voltage generating circuit 42 can be madesmall.

Similarly to the first and second embodiments, this content addressablememory includes search data input circuit 4 and control circuit 8, andthe respective internal operation cycles in the search cycle are setunder the control of control circuit 8 (based on clock signal CLK).

FIG. 12 specifically shows a construction of match amplifier 40 shown inFIG. 11. FIG. 12 representatively shows a construction of the matchamplifier arranged for match line ML[0]. Match amplifiers having thesame constructions as that in FIG. 12 are arranged for the other matchlines, respectively.

Match amplifier 40 shown in FIG. 12 differs from the match amplifiershown in FIG. 8 in the following construction. P-channel MOS transistorsPQ10 and PQ11 are arranged in series between the power supply node andcorresponding match line ML (ML[0]). MOS transistor PQ10 receives biasvoltage BIAS_P on its gate, and MOS transistor PQ11 receives a pull-upinstructing signal MLPU_n on its gate. Other constructions of matchamplifier 40 shown in FIG. 12 as we as the constructions of respectiveentries ERY (ERY0-ERYn) are the same as those shown in FIG. 8.Corresponding portions bear the same reference numbers, and descriptionthereof is not repeated.

MOS transistor PQ10 supplies a current of a constant magnitude accordingto bias voltage BIAS_P. This current is smaller than a one-bit pull-outcurrent, In, that flows through one-bit unit cell out of the unit cellsin one entry when the unit cell is conductive and is larger than aleakage current IOFF that flows when all the unit cells in thecorresponding entry are non-conductive.

One-bit pull-out current In indicates a current flowing through a seriesconnection of the conductive transistors in the unit cell in the missstate, and does not includes a leakage current in a path of anon-conductive transistor(s). In the unit cell in the miss state,one-bit pull-out current In and the off-leakage current flow. However,this one-bit pull-out current In is much larger than a current flowingthrough the series connection of the non-conductive transistors (TR1 andTR2, or TR3 and TR4). In the unit cell in the miss state, the seriesconnection of the non-conductive MOS transistors (TR1 and TR2) in theunit cell in the miss state has a combined resistance value smaller thanthat of the series connection of the MOS transistors in the unit cell inthe match state, and causes a leakage current much smaller than anoff-leakage current Ioff. In the following description, it is assumedthat a one-bit miss current Imiss including an off-leakage component hasthe same magnitude as one-bit pull-out current In unless otherwisespecified. One-bit miss current Imiss is a current flowing through theunit cell in the miss state.

Precharge voltage VML is used as the comparison reference voltage forsensing the voltage level of the match line. Differential amplifiercircuit 12 may be formed of either the differential amplifier circuit ofthe current mirror type shown in FIG. 6 or the differential amplifiercircuit of the cross-coupled type (the cross-coupled, latch type senseamplifier) shown in FIG. 10.

FIG. 13 is a timing chart representing a search operation of a contentaddressable memory shown in FIG. 12. Referring to FIG. 13, the searchoperation of the content addressable memory shown in FIG. 12 will now bedescribed:

At time T1, the search cycle starts. When the search cycle starts,precharge instructing signal PRE_n is driven to the L level andprecharge transistor 14 turns conductive. Accordingly, match line ML ischarged, and has the voltage level driven to the intermediate voltagelevel of precharge voltage VML.

When the precharge operation is completed, precharge instructing signalPRE_n is made inactive at time T2. Subsequently or concurrently, searchlines SL and /SL are driven to the levels of the power supply voltageand ground voltage according to the search data. Also, pull-upinstructing signal MLPU_n attains the L level. Thereby, MOS transistorPQ11 is turned on to supply a pull-up current Ip from the power supplynode to corresponding match line ML via MOS transistors PQ10 and PQ11.Pull-up current Ip is smaller than one-bit pull-out current In, and islarger than all-bit off-leakage current IOFF. Therefore, when the searchdata does not match the stored data in the entry, corresponding matchline ML is discharged, and the voltage level thereof lowers belowprecharge voltage VML. The match line in the match state has the voltagelowering due to the leakage current compensated for by pull-up currentand has the voltage level raised as will be described later.

At time T3, the active period of search lines SL and /SL expires. Also,the pull-up current supplying period expires, and pull-up instructingsignal MLPU_n attains the H level to stop the supply of the pull-upcurrent to the match line. At time T3, the determination cycle starts,and isolation instructing signal MLI attains the L level. Also, matchamplifier activating signal MAE is made active, and latch instructingsignal LAT attains the H level. Therefore, at the sense nodes (+ and −)of differential amplifier circuit 12, the voltage of corresponding matchline ML and precharge voltage VML are confined, and differentialamplifier circuit 12 performs the differential amplification of thevoltages on the sense nodes. In the miss state (mismatch state), theoutput signal of differential amplifier circuit 12 attains the L levelof the ground voltage, and search result indicating signal ML_OUT is setto the L level via latch 16.

At time T4, the period of the data comparison and latch amplifier outputends. Thereby, match amplifier activating signal MAE and latchinstructing signal LAT are driven to the L level. Thereby, latch 16enters the latch state, and differential amplifier circuit 12 becomesinactive. At this time, isolation instructing signal MLI is at the Llevel, and isolation gate circuit 30 is off. In this state, the searchoperation is already completed, and search lines SL and /SL are both atthe ground voltage level. Therefore, the discharging path of match lineML is not present, and only the off-leakage current of unit cell UC inthe entry is present. Thereby, the match line substantially is kept atthe voltage level attained at time T3 for a period between times T3 andT5.

At time T5, a next search cycle starts, so that precharge instructingsignal PRE_n attains the L level again, and match line ML is driven tothe level of precharge voltage VML.

At time T6, the voltage levels of search lines SL and /SL are setaccording to the search data. Also, pull-up instructing signal MLPU_n ismade active and pull-up current Ip is supplied to match line ML. In thematch state where the search data matches the stored data of thecorresponding entry, match line ML allows the flow of only off-leakagecurrent IOFF of all the bits of unit cells UC in the corresponding entry(=m·Ioff where m is the number of unit cells in the entry). This all-bitoff-leakage current IOFF is compensated for by pull-up current Ip(IOFF<In), and pull-up current Ip raises the voltage level of match lineML in the match state.

At time T7, match amplifier activating signal MAE becomes active, andisolation instructing signal MLI attains the L level. Accordingly,isolation gate circuit 30 is turned non-conductive, and differentialamplifier circuit 12 executes the amplification in the charge confiningstate. In the match state, search result indicating signal ML_OUTsupplied from latch 16 attains the H level of the power supply voltagelevel.

The voltage level of match line ML in the match state is higher thanthat of precharge voltage VML. In the next search cycle, the prechargingby precharge transistor 14 drives match line ML at the raised voltagelevel to the level of precharge voltage VML. It is sufficient forIntermediate voltage generating circuit 42 (see FIG. 11) generatingintermediate voltage VML to include a construction for discharging theraised potential of match line ML upon rise of the match line voltage.Thereby, even when match line ML has the voltage level raised aboveprecharge voltage VML, it can be reliably set to precharge voltage VMLthrough the precharging.

The pull-up operation performed on match line ML by MOS transistor PQ10can be stopped after isolation gate circuit 30 is turned off to confinethe charges. Therefore, even when match line ML is in the match state,the match line does not fully swing to the level of power supply voltageVDD, and the current consumption can be small.

FIG. 14 shows an example of a construction of bias voltage generatingcircuit 45 shown in FIG. 11. In FIG. 14, bias voltage generating circuit45 includes a replica entry 50 having substantially the sameconstruction as the path for discharging the match line of one entry ERY. Replica entry 50 includes replica unit cells equal in number to unitcells UC included in one entry ERY of the memory cell array. One replicaunit cell UCs is set to the miss state or in the mismatch state, and theother replica unit cells UCh are set to the match state. Replica unitcells UCs and UCh are coupled to a common replica match line RML.Replica match line RML supplies a current via a diode-connectedP-channel MOS transistor P60. P-channel MOS transistor P60 functions asa current/voltage converting element to produce bias voltage BIAS_P onits gate.

Each of replica unit cells UCs and UCh has transistors N61, N62, N63 andN64 of the same size (the same ratio of a channel width to a channellength) as transistors TR1, TR2, TR3 and TR4 in the match linedischarging path of unit cell UC in entry ERY , respectively, and causesa current flow of the same magnitude as unit cell UC. In replica unitcell UCs in the miss state, MOS transistors N61 and N62 arenon-conductive, and MOS transistors N63 and N64 are conductive.

In replica unit cell UCs in the miss state or the mismatch state,therefore, a current of the same magnitude as one-bit pull-out currentIn of unit cell. UC is discharged from replica match line RML to theground node. In replica unit cell UCh, one of MOS transistors N61 andN62 is non-conductive, and the other is conductive. Also, one of MOStransistors N63 and N64 is conductive, and the other is non-conductive.In this discharging path, one of the MOS transistors is conductive, andthe other is non-conductive, whereby the same state as the match statein unit cell UC of entry ERY can be achieved. In unit cell UC in thematch state, one of MOS transistors TR1 and TR2 is off, and one of MOStransistors TR3 and TR4 is off. Therefore, replica unit cell UChdischarges the current of the same magnitude as off-leakage current Ioffin unit cell UC.

Therefore, MOS transistor P60 supplies a current of (In+(m−1)·Ioff),where m represents a total number of replica unit cells UCh and UCs inthe replica entry, or the number of unit cells in one entry ERY .

MOS transistor P60 is larger in size (the ratio of the channel width tothe channel length) than MOS transistor PQ10. Therefore, current Ippassing through MOS transistor PQ10 is smaller than the current passingthrough the match line corresponding to the entry in the one-bit missstate. Using replica entry 50, replica unit cell UCs of one bit is setto the miss state, and other replica unit cells UCh are set to the matchstate. Thereby, replica match line RML can cause a flow of the currentof the same magnitude as the current that passes through the match linein the one-bit miss state.

MOS transistors P60 and PQ10 form the current mirror circuit, and thetransistor sizes (current supplying capability) of these transistors areadjusted. Thereby, match line ML can cause a flow of the current smallerthan one-bit pull-out current In and larger than leakage current IOFFflowing in the all-bit off state. Thereby, the comparison and searchoperation can be performed using precharge voltage VML as comparisonreference voltage.

In the above description, a current smaller than one-bit pull-outcurrent In flows as pull-up current Ip of the match line. However, thecurrent value of the pull-up current may be restricted to be smallerthan that of current Imiss flowing through the match line in the one-bitmiss state. The bias voltage generating circuit shown in FIG. 14produces bias voltage BIAS_P is configured to satisfy this condition.

[Modification]

FIG. 15 shows a construction of a modification of the third embodimentaccording to the invention. The content addressable memory shown in FIG.15 differs from the content addressable memory shown in FIG. 14 in thefollowing construction. In each match amplifier 40, a capacitanceelement CQ (CQ0, CQ1, . . . ) is arranged neighboring to isolation gate30 b of isolation gate circuit 30 and spaced from differential amplifiercircuit 12 with isolation gate circuit 30 in between. These capacitanceelements CQ are commonly supplied with precharge voltage VML via aP-channel MOS transistor 55 that is selectively turned on according toprecharge instructing signal PRE_n. Other constructions of the contentaddressable memory shown in FIG. 15 are the same as those of the contentaddressable memories shown in FIGS. 12 and 14. Corresponding portionsare allotted the same reference numerals, and description thereof is notrepeated.

In the construction of the content addressable memory shown in FIG. 15,MOS transistors 14 and 55 are made conductive when precharge instructingsignal PRE_n is active. Therefore, precharge voltage VML is supplied tocapacitance elements CQ (CQ1, CQ2, . . . ) in parallel with theoperation of precharging the match lines ML (ML[0]-ML[n]) to the levelof precharge voltage VML. When precharge instructing signal PRE_n isinactive, a charged voltage VML_i of capacitance elements CQ (CQ1, CQ2,. . . ) is at the same level as the precharge voltage on the match linewhen the precharging is completed. In the search operation, prechargevoltage VML_i of capacitance elements CQ (CQ1, CQ2, . . . ) is confinedby isolation gate circuit 30, and is used as a comparison referencevoltage VML_ref for comparison with the potential of the correspondingmatch line. When differential amplifier circuit 12 operates, thiscomparison reference voltage VML_ref can be kept at substantially thesame level as the precharge voltage of the match line. Even whenvariations occur in voltage level of precharge voltage VML produced bythe intermediate voltage generating circuit in the memory, the prechargevoltage level attained by the precharge can be reliably used as thecomparison reference voltage in each search cycle. Thereby, a sufficientmargin can be ensured for the amplifying operation (sense operation) ofdifferential amplifier circuit 12.

Even when the voltage level of precharge voltage VML produced by theintermediate voltage generating circuit lowers due to the prechargeoperation, it can be restored to the original voltage level. The variousmethods for restoring the voltage level can be considered depending onthe construction of the intermediate voltage generation circuit.Therefore, precharge voltage VML attained at the end of the prechargingmay be different in level from precharge voltage VML attained at thestart of the sense operation (differential amplification). However,precharge voltage VML is held in capacitance elements CQ (CQ1, CQ2, . .. ) when the precharging is completed, and such voltage difference ofthe precharge potential can reliably avoided to set comparison referencevoltage VML_ref to the same voltage level as the precharge voltage ofthe match line.

A signal line 57 is arranged commonly to capacitance elements CQ0, CQ1,. . . , and a precharge voltage transistor 55 is shared among matchamplifiers 40, so that match amplifiers 40 can use the same comparisonreference voltage level. Accordingly, in the determination cycle, matchamplifier 40 can perform accurately the differential amplification,using the comparison reference voltage at the same level so that thedeviations in definition timing of determination result can be small.

According to the third embodiment of the invention, as described above,the same voltage level as the precharge voltage level is used as thecomparison reference voltage. Therefore, the occupation area and powerconsumption of the internal voltage generating circuit can be reduced.Further, by supplying the pulling-up current in activation of the matchline, the voltage level of the match line can be reliably set higher orlower than the precharge voltage level depending on the search result,so that the search and determination operation can be accuratelyperformed. Provision of isolation gate circuit 30 can reduce the pull-upperiod of the match line, and can reduce the voltage amplitude of thematch line so that the current consumption can be small.

The capacitance element holds the precharge voltage attained at the endof the match line precharging. Therefore, the determination operationcan be performed using the precharge voltage level of the match line asthe comparison reference voltage, and thus can be performed with a largenoise margin and high accuracy.

The same effects as those of the first to third embodiments can also beprovided.

Fourth Embodiment

FIG. 16 shows a main portion of a content addressable memory accordingto a fourth embodiment of the invention. The content addressable memoryshown in FIG. 16 differs from the content addressable memory shown inFIG. 15 in the following construction. An N-channel MOS transistor 60 isarranged corresponding to each of match lines ML (ML[0]-ML[n]) fordischarging the corresponding match line to the ground voltage level inresponse to a discharge instructing signal DIS. Other constructions ofthe content addressable memory shown in FIG. 16 are the same as those ofthe content addressable memory shown in FIG. 15. Corresponding portionsbear the same reference numbers, and description thereof is notrepeated.

FIG. 17 is a timing chart representing a search operation of the contentaddressable memory shown in FIG. 16. Referring to FIG. 17, the operationof the content addressable memory shown in FIG. 16 will now bedescribed.

At time T1, the search cycle starts. When the search cycle starts attime T1, precharge instructing signal PRE_n is first activated.Accordingly, match lines ML (ML[0]-ML[n]) are precharged to prechargevoltage VML at the intermediate voltage level via correspondingprecharge transistors 14, respectively.

At time T2, the search line activation and the pull-up operation areperformed, pull-up instructing signal MLPU_n becomes active and searchlines SL and /SL are supplied with the search data. When thecorresponding entry is in the mismatch state, the operation similar inthe third and fourth embodiments is performed. Thus, match line ML isdischarged via the unit cell in the miss state, and the voltage levelthereof lowers from precharge voltage VML.

At time T3, the search line activation and pull-up operation arecompleted, and sensing the data and outputting the result of sensing areperformed. Specifically, at time T3, pull-up instructing signal MLPU_nis driven to the H level, and both search lines SL and /SL are driven tothe ground voltage level. Also, match amplifier activating signal MAEbecomes active, and latch instructing signal LAT attains the H level.Discharging of match line ML ends, isolation gate circuit 30 enters thecut-off state in response to isolation instructing signal MilDifferential amplifier circuit 12 performs the differentialamplification according to the charge confining manner, and latch 16produces output ML_OUT. In the mismatch state, search result indicatingsignal ML_OUT is at the ground voltage level.

When search result indicating signal ML_OUT is in the definite state,discharge instructing signal PRE_n becomes active at time T4, and alsomatch amplifier activating signal MAE becomes inactive. Further, latchinstructing signal LAT attains the L level, and latch 16 enters thelatch state. In this state, isolation gate circuit 30 is in the shut-offstate. Match line ML is discharged to the ground voltage level via MOStransistor 60.

A next search cycle starts at time T5. According to the activation ofprecharge instructing signal PRE_n, match line ML is driven from theground voltage level to the level of precharge voltage VML. Thereafter,the search line is activated and the pull-up current is supplied at timeT6. The pull-up current raises the voltage level of match line ML in thematch state to the level above precharge voltage VML.

At time T7, a cycle of determining and outputting the data search resultis executed. In this cycle, isolation gate circuit 30 attains theshut-off state in response to isolation instructing signal MLI being atthe L level. Match amplifier activating signal MAE becomes active, andfurther latch instructing signal LAT attains the H level. Search resultindicating signal ML_OUT is driven to the H level indicating the matchstate.

At time T8, the operation of determining the comparison result andoutputting the search result ends, and match amplifier activating signalMAE is made inactive. Also, latch instructing signal LAT attains the Llevel, and search result indicating signal ML_OUT is kept in the latchedstate at the H level. At time T8, discharge instructing signal DISattains the H level again, and discharging transistors 60 are turned onto discharge the respective match lines ML. Match line ML in the matchstate is at a voltage level higher than precharge voltage VML, and isdriven by the discharging to a voltage level (the ground voltage levelin FIG. 17) lower than precharge voltage VML.

Precharging is performed in the search cycle starting at time T9 todrive match line ML in the match state to the level of precharge voltageVML.

As indicated by an alternate long and short dash line in FIG. 17, wheredischarge instructing signal DIS is produced in a one-shot pulse form,match line ML in the match state may be configured to stop thedischarging at a voltage level higher than ground voltage level. In thenext search cycle, the match line in the match state is precharged tothe precharge voltage level from the voltage level between prechargevoltage VML and ground voltage GND.

By using this discharging transistor 60, the precharging of match lineML is always performed in the charging direction and therefore is thepulling-up operation. Thus, there is no need in the intermediate voltagegenerating circuit for generating precharge voltage VML to provide aconstruction for discharging the voltage level of precharge voltage VML,to hold a predetermined voltage level, and the circuit configuration canbe simple. For example, a circuit that produces precharge voltage VMLcan be implemented by a circuit construction similar to a feed-back typeinternal voltage down converter (VDC) configured by a comparison circuitand a current drive transistor. Alternatively, the circuit for producingprecharge voltage VML may be implemented by utilizing a source-followermode operation of an N-channel MOS transistor to hold a gate potentialof the source-follower transistor at a predetermined voltage level.

In this case, a circuit for lowering the voltage level of intermediatevoltage VML upon rising thereof is not required in the feed-back typecontrol circuit. When the source-follower transistor is utilized, asource-follower transistor (P-channel MOS transistor) dischargingprecharge voltage VML is not required as the source-follower transistor.Therefore, the circuit construction can be simple, and the currentconsumption of the circuit for generating precharge voltage VML can bereduced.

FIG. 18 schematically shows a construction of a control circuit used inthe fourth embodiment. In FIG. 18, control circuit 8 includes commanddecoder 20 that decodes externally supplied command CMD insynchronization with clock signal CLK, and precharge activating circuit22 that produces precharge activating signal PRE_n according to searchoperation instruction EN supplied from command decoder 20 and clocksignal CLK.

Control circuit 8 further includes a search line drive activatingcircuit 64 for producing search line activating signal SLEN, delaycircuit 26 for delaying search operation instruction EN, a matchamplifier activating circuit 65 for producing match amplifier activatingsignal MAE, and latch instructing signal LAT according to the outputsignal of delay circuit 26, and a discharge control circuit 68 forproducing the discharge instructing signal according to the outputsignal of delay circuit 26.

When search operation instruction EN is active, search line driveactivating circuit 64 maintains search line activating signal SLEN foractivating the search line in the active state during a period of the Llevel of clock signal CLK.

Delay circuit 26 delays search operation instruction EN by one clockcycle period. When the output signal of delay circuit 26 is active,match amplifier activating circuit 66 drives match amplifier activatingsignal MAE and latch instructing signal LAT to the H level in responseto the H level of clock signal CLK.

Discharge control circuit 68 activates, according to the output signalof delay circuit 26, discharge instructing signal DIS in synchronizationwith the falling of clock signal CLK.

Discharge control circuit 68 may drive discharge instructing signal DISto the L level while clock signal CLK is at the L level, or may drivedischarge instructing signal DIS to the H level in a one-shot pulse formin synchronization with the falling of clock signal CLK (in a mannercorresponding to a waveform represented by alternate long and short dashline in FIG. 17).

According to the fourth embodiment of the invention, as described above,each match line is provided with the discharge transistor for drivingthe corresponding match line to the ground voltage level when the searchoperation ends. Therefore, the precharge voltage generating circuit canbe configured of a charging type circuit, and the circuit constructioncan be simple, and the current consumption can be small.

In addition, the effects similar to those in the first to thirdembodiments can be achieved.

Fifth Embodiment

FIG. 19 shows a main portion of a content addressable memory accordingto a fifth embodiment of the invention. The content addressable memoryshown in FIG. 19 differs from the content addressable memory shown inFIG. 15 in the following construction. Match amplifier 40 includesP-channel MOS transistors PQ70 and PQ11 as well as a capacitance element70 serving as a pull-up current supplying source for match line ML(ML[0]-ML[n]). P-channel MOS transistor PQ70 is turned on in response toa charge instructing signal CHA_n. Capacitance element 70 is charged tothe level of power supply voltage VDD through conductive P-channel MOStransistor PQ70. The accumulated charges of capacitance element 70 aresupplied to corresponding match line ML via P-channel MOS transistorPQ11 that is selectively made conductive according to pull-upinstructing signal MLPU_n.

Other constructions of the content addressable memory shown in FIG. 19are the same as those of the content addressable memory shown in FIG.15. Corresponding portions are allotted the same reference numerals, anddescription thereof is not repeated.

FIG. 20 is a timing chart representing the search operation of thecontent addressable memory shown in FIG. 19. Referring to FIG. 20,description will now be given on the search operation of the contentaddressable memory shown in FIG. 19.

At time T1, the search cycle for the search operation starts. When thesearch cycle starts, precharge instructing signal PRE_n is firstactivated. Accordingly, each MOS transistor 14 for precharging turnsconductive to precharge corresponding match line ML to the level ofprecharge voltage VML.

At time T2, the search line is made active. Concurrently with theactivation of the search line, pull-up instructing signal MLPU_n attainsthe L level. Accordingly, MOS transistor PQ11 is turned on to supply theaccumulated charges of capacitance element 70 to the match line, andaccordingly the voltage level thereof rises. When match line ML is inthe mismatch state, one-bit pull-out current In flows through one bit ofunit cell UC in the miss state in the entry, and the voltage level ofmatch line ML lowers.

In the pull-up operation of match line ML, the accumulated charges ofcapacitance element 70 are merely supplied to the corresponding matchline, and the power supply node is isolated from the match line.Therefore, the pull-up current cannot flow from the power supply node tothe ground node, and the current consumption can be small.

At time T3, the match line pulling-up operation ends, the activation ofthe search line ends, and the search result is determined and read out.Specifically, at time T3, pull-up instructing signal MLPU_n is driven tothe H level, and isolation instructing signal MLI is set to the H levelso that isolation gate circuit 30 attains the non-conductive state.Thus, each differential amplifier circuit 12 performs the differentialamplification according to the charge confining scheme in response tothe activation of match amplifier activating signal MAE. After theamplifying operation, signal ML_OUT indicative of the determinationresult is output via latch 16.

Concurrently with the amplifying operation of differential amplifiercircuit 12, charge instructing signal CHA_n is activated and thecharging operation of capacitance element 70 is performed.

At time T4, match amplifier activating signal MAE is made inactive, andlatch instructing signal LAT is driven to the L level. Accordingly,latch 16 attains the latching state and one search cycle completes.

In the search cycle starting at time T5, the operation is performed inthe case when match line ML is in the match state. In this case,capacitance element 70 supplies its accumulated charges via MOStransistor PQ11 to match line ML at time T6 after completion of theprecharge operation by precharge transistor 14. Thereby, match line MLis held at the level of the charged voltage. When match line ML is inthe match state, the raised voltage level is merely set byredistribution of the accumulated charges of capacitance element 70. Acapacitance ratio between capacitance element 70 and the loadcapacitance of match line ML determines the voltage level of the matchline, and the voltage amplitude of the match line can be sufficientlysmall.

When the activation of search lines SL and /SL and the pull-up operationof the match line are completed, isolation instructing signal MLIattains the L level at time T7, and isolation gate circuit 30 attainsthe non-conductive state. Then, match amplifier activating signal MAE ismade active to cause the amplifying operation by the match amplifier.Then, latch instructing signal LAT attains the H level, and latch 16produces signal ML_OUT at the H level indicating the match state. Thecharge operation of capacitance element 70 is performed via MOStransistor PQ70.

Match line ML is kept at the voltage level attained through the chargingvia capacitance element 70 in the last cycle starting at time T6. When aprecharge operation is performed in a next search cycle starting at atime T9, the circuit for producing precharge voltage VML sets match lineML to the intermediate voltage level of precharge voltage VML.Therefore, the timing for charging capacitance element 70 may besufficient to be during a period of the H level of pull-up instructingsignal MLPU_n. The charge amount required for charging capacitanceelement 70 is sufficient to be the charge amount for causing a voltagerising to the voltage level to be sensible by differential amplifiercircuit 12 on corresponding match line ML. Therefore, the chargingperiod of capacitance element 70 can be laid within one search cyclesufficiently.

For the circuit for producing charge instructing signal CHA_n, it issufficient to use the construction in which in the control circuit 8shown in FIG. 8, match amplifier activating circuit 66 activates chargeinstructing signal CHA_n at the same timing as match amplifieractivating signal MAE. Alternatively, charge instructing signal CHA_nmay be activated at the same timing as the discharge instructing signalfor discharging the match line to the voltage level as describedpreviously.

In the construction shown in FIG. 19, each match line ML may be providedwith a discharging transistor (60) for discharging corresponding matchline ML to the ground voltage level according to a discharge instructingsignal (DIS), similarly to the construction shown in FIG. 16. In thiscase, the circuit for producing precharge voltage VML is merely requiredto charge the match line. Therefore, the intermediate voltage generatingcircuit for generating precharge voltage VML can have a simpleconstruction, and therefore, the simple construction of the intermediatevoltage generating circuit and the reduced current consumption can beachieved similarly to the fourth embodiment.

According to the fifth embodiment of the invention, the accumulatedcharges of the capacitance element are used when pulling up the matchline in the search operation. In the pull-up operation of the matchline, therefore, a current path from the power supply node to the groundnode is cut off to reduce the current consumption. Further, in the fifthembodiment, the same effects as the first to fourth embodiments can beachieved.

Sixth Embodiment

FIG. 21 shows a main portion of a content addressable memory accordingto a sixth embodiment of the invention. The content addressable memoryshown in FIG. 21 differs from the content addressable memory shown inFIG. 14 in the following construction. An isolation gate circuit is notprovided for differential amplifier circuit 12. Reference voltage VREFis normally applied to the negative input of the differential amplifier.Match line ML is always coupled to the positive input of thedifferential amplifier. In match amplifier 40, P-channel MOS transistorsPQ10 and PQ72 for charging the match line are connected in seriesbetween the power supply node and match line ML. For discharging matchline ML to the ground voltage level, N-channel MOS transistor 60 isarranged between match line ML and the ground node. MOS transistors PQ72and 60 receive discharge instructing signal DIS at their gates.P-channel MOS transistor PQ10 receives bias voltage BIAS_P at its gate.This bias voltage BIAS_P is supplied from intermediate voltagegenerating circuit 45 having the same construction as that shown in FIG.14.

Since intermediate voltage generating circuit 45 has the sameconstruction as that shown in FIG. 14, corresponding portions areallotted the same reference numerals, and description thereof is notrepeated. The construction of unit cell UC of entry ERY in the memorycell array as well as other constructions of latch amplifier 40 are thesame as those in the content addressable memory shown in FIG. 14 so thatcorresponding portions are allotted the same reference numerals, anddescription thereof is not repeated.

In the construction shown in FIG. 21, the replica entry in intermediatevoltage generating circuit 45 is in such a state that one bit of unitcell UCs is in the mismatch state, and the other unit cells UCh are inthe match state. Through replica match line RML, MOS transistor P60causes a current flow of a sum of one-bit pull-out current In andoff-leakage currents of ((M−1)·Ioff), where m represents a total numberof replica unit cells UCh and UCs. MOS transistor P60 has a larger sizethan MOS transistor PQ10. Therefore, current Ip flowing through MOStransistor PQ10 is made smaller than one-bit pull-out current In,one-bit miss current Imiss, or a current of (In+(M−1)·Ioff) passingthrough the match line in the one-bit miss state. Reference voltage VREFis set to the voltage level of (VDD/4).

FIG. 22 is a timing chart representing a search operation of the contentaddressable memory shown in FIG. 21. Referring to FIG. 22, descriptionwill now be given on a search operation of the content addressablememory shown in FIG. 21.

At or before time T1, discharge instructing signal DIS is at the Hlevel. MOS transistor 60 is on, and match line ML is kept at the levelof ground voltage GND. In this state, MOS transistor PQ72 is off, andthe pull-up operation of match line ML is not performed.

At time T1, the search operation cycle starts. In the search cycle,match line ML is precharged to the ground voltage level so that searchlines SL and /SL are driven to the voltage level corresponding to thesearch data simultaneously with the start of the search cycle. When thesearch cycle starts, discharge instructing signal DIS attains the Llevel, and MOS transistors 60 and PQ72 are turned on. Accordingly, eachmatch line ML is supplied with pull-up current Ip via MOS transistorsPQ10 and PQ72, and the voltage level thereof rises. Also, the voltagelevel of match line ML, in the mismatch state lowers because itsdischarge current is larger than pull-up current Ip.

At time T3, match amplifier activating signal MAE is made active, andlatch instructing signal LAT attains the H level (shown in FIG. 22), anddetermination on the voltage level of match line ML and output of aresult of the determination are performed. When mismatch occurs, latch16 generates the signal ML_OUT at the L level.

At time T4, when the determination and output of the search result arecompleted, match amplifier activating signal MAE is made inactive tostop the differential amplification of differential amplifier circuit12. Latch instructing signal LAT attains the L level to set latch 16 tothe latch state. At the time T4, discharge instructing signal DIS isdriven to the H level according to deactivation of match amplifieractivating signal MAE. Responsively, supply of pull-up current Ip stops,and match line ML is discharged to the ground voltage level.

In the search cycle starting at time T5, the states of search lines SLand /SL are set according to the next search data again. The prechargeoperation of match line ML ends so that MOS transistors PQ10 and PQ72supply pull-up current Ip. When match line ML is in the match state, adischarge path of match line ML is not present, and pull-up current Ipraises the voltage level of the match line in the match state.

At time T7, when the potential of match line ML rises above referencevoltage VREF, match amplifier activating signal MAE is made active, andthe determination and output of the search result are performed. Duringthis determination and output of the search result, pull-up current Ipis likewise supplied so that the voltage level of match line ML rises.

At time T8, the determination and output of the search result end, andmatch amplifier activating signal MAE is made inactive. Dischargeinstructing signal DIS is driven to the H level so that the supply ofpull-up current Ip to match line ML stops, and match line ML isdischarged to the ground voltage level.

In the content addressable memory shown in FIG. 21, a prechargingtransistor (14) shown in FIG. 14 is not employed. At the start of thesearch cycle, therefore, it is not necessary to precharge the pluralityof match lines (only supply of the pull-up current of a restrictedcurrent value is performed), so that it is possible to reduce atransient current due to the simultaneous precharging of the pluralityof match lines.

Reference voltage VREF is set to the level of (voltage VDD)/4 or lower(VDD/4 in FIG. 22). Therefore, the pull-up level of the voltage in thehigh level direction on match line ML in the match state is set to thevoltage level of VDD/2 or lower (VDD/2 in FIG. 22). In differentialamplifier circuit 12, the amplitudes of the input signals at the highlevel and at the low level is made equal with respect to referencevoltage VREF. The signal amplitude of match line ML can be set to VDD/2or lower, and the current consumption can be reduced.

[Modification]

FIG. 23 shows a construction of a modification of bias voltagegenerating circuit 45 according to the sixth embodiment of theinvention. In FIG. 23, bias voltage generating circuit 45 includes areplica entry 80 including the unit cells the same in construction asthe unit cells in entry ERY all set to the match state in the memorycell array. Replica entry 80 includes such replica unit cells UCh thatall bits are set to the match state. Each replica unit cell UCh causes aflow of off-leakage current Ioff.

Replica unit cell UCh includes a series connection of MOS transistorsN61 and N62 as well as a series connection of MOS transistors N63 andN64. In each series connection, one of the MOS transistors is kept off,as in the unit cell in the match state. In each series connection, anoff-leakage current flows. In one replica unit cell UCh, a total currentof the off-leakage currents in the respective series connections of thereplica unit cell simulates the off-leakage current in one unit cell inthe match state.

MOS transistors N61 and N62 are the same in size as transistors TR1 andTR2 of unit cell UC shown in FIG. 21, respectively. MOS transistors N63and N64 have the same sizes as transistors TR3 and TR4 in unit cell UCshown in FIG. 21, respectively. Replica unit cells UCh of the replicaentry 80 are commonly coupled to a replica match line RMLa.

Bias voltage generating circuit 45 further includes a one-bit replicaunit cell 82. This one-bit replica unit cell 82 is in the same state asunit cell UCs in the miss state. In FIG. 23, one-bit replica unit cell82 includes a series connection of MOS transistors NT61 and NT62 as wellas a series connection of MOS transistors NT63 and NT64. These MOStransistors NT61 and NT62 are set off, and MOS transistors NT63 and NT64are set on. These MOS transistors NT61 and NT62 have the same sizes astransistors TR1 and TR2 of unit cell UC, respectively. MOS transistorsNT63 and NT64 have the same sizes as transistors TR3 and TR4 of the unitcell, respectively. Therefore, one-bit miss current Imiss flows througha one-bit replica match line MLU of one-bit replica unit cell 82.

Bias voltage generating circuit 45 further includes a P-channel MOStransistor P601 that has a gate and a drain interconnected to each otherand supplies a current from a power supply node to replica match lineRMLa, and a P-channel MOS transistor P602 supplying a current Ia to asignal line 85. MOS transistors P601 and P602 form a current mirrorcircuit.

Bias voltage generating circuit 45 further includes a P-channel MOStransistor P604 supplying a current to one-bit replica unit cell 82, anda P-channel MOS transistor P603 supplying a current Ib to signal line85. MOS transistor P604 has a gate and a drain interconnected to eachother. MOS transistors P603 and P604 form a current mirror circuit.

MOS transistors P601 and P602 are the same in size (ratio of the channelwidth to the channel length), and supply currents of the same magnitudeof (m·Ioff=Ia, where m represents the number of replica unit cells UChand is equal to the number of unit cells in one entry ERY ). MOStransistor P603 has a smaller size than MOS transistor P604. Therefore,P-channel MOS transistor P603, produces a mirror current smaller thanone-bit miss current Imiss nearly equal to In (Ib<Imiss). Signal line 85conducts a current of a sum of currents Ia and Ib flowing through MOStransistors P602 and P603, respectively. The current flowing throughsignal line 85 is expressed by the following equation:

Ia+Ib=m·Ioff+Ib

Transistors P602 and P603 can have the sizes adjusted to pass, throughsignal line 85, a current smaller than the current that flows from thematch line via the entry in the one-bit miss state.

Imiss+(m−1)·Ioff>Ib+m·Ioff>m·Ioff

The following inequation is derived from the above relation:

Imiss−Ioff>Ib

Current Ib supplied by MOS transistor P603 is set to satisfy the aboverelationship. The current flowing through signal line 85 can be setlarger than the off-leakage current of the entry in the match state andsmaller than the discharging current (one-bit miss current) of the entryin the one-bit miss state. The upper limit of current Ib is equal to adifference between current In flowing through a path of the conductivetransistor of the unit cell in the miss state in the entry in theone-bit miss state and off-leakage current Ioff/2 flowing through theseries connection of the non-conductive transistors.

Bias voltage generating circuit 45 further includes an N-channel MOStransistor N601 discharging the current from signal line 85, anN-channel MOS transistor N602 and a P-channel MOS transistor P605supplying a current to MOS transistor N602. MOS transistor N601 has agate and a drain connected to each other. MOS transistors N602 and N601form a current mirror circuit. MOS transistor P605 has a gate and adrain connected to each other.

MOS transistors N601 and N602 have the same size (the same ratio of thechannel width to the channel length). Therefore, a current Ic flowingthrough MOS transistor N602 is equal in magnitude to the current flowingthrough signal line 85. Thus, MOS transistor P605 passes the current ofthe same magnitude as that flowing through MOS transistor N602. MOStransistor P605 has a gate and a drain connected to each other, and hasa current/voltage converting function to produce bias voltage BIAS_P onits gate. MOS transistor PQ10 included in match amplifier 40 receivesbias voltage BIAS_P at its gate.

MOS transistors P605 and PQ10 are the same in size. Therefore, a currentId flowing to match line ML via MOS transistors PQ10 and PQ70 is equalin magnitude to the current flowing through signal line 85.

Thus, each match line ML can be supplied with the current smaller thanthe current flowing through the match line in the one-bit miss(mismatch) state, and larger than the off-leakage current of the entryin the all-bit match state. Since the replica entry is used, variationsin transistor parameter of the unit cells of the entry storing the datacaused during the manufacturing can be reflected on the replica unitcells of the replica entry. Accordingly, the pull-up current andprecharge current of a desired magnitude can be accurately supplied.

In the construction shown in FIG. 23, the number of replica unit cellsUCh included in replica entry 80 may be (m−1), which is smaller by onethan the number m of the unit cells included in entry ERY . Replicaentry 80 and one-bit replica unit cell 82 can produce more precisely acurrent corresponding to the current that flows through the match lineassociated with the entry in the one-bit mismatch state includingone-bit miss state unit cell.

FIG. 24 schematically shows a construction of control circuit 8 of thecontent addressable memory according to the sixth embodiment of theinvention. In FIG. 24, control circuit 8 includes command decoder 20 fordecoding externally supplied command CMD, a frequency divider 90 forfrequency-dividing clock signal CLK according to search operationinstruction EN supplied from command decoder 20, and a search data inputcontrol circuit 92 for producing a latch enable signal LIEN for searchdata input circuit 4 according to search operation instruction ENreceived from command decoder 20 and a frequency-divided clock signalBCLK received from frequency divider 90.

Search data input circuit 4 is formed of a flip-flop circuit 94, and isconfigured to take in and latch search data SD according to activationof latch enable signal LTEN, for driving a search line group (searchdata bus) SLG according to the taken search data.

Control circuit 8 includes delay circuit 26, match amplifier activatingcircuit 66 and discharge control circuit 68. Delay circuit 26 delayssearch operation instruction EN by one clock cycle. Match amplifieractivating circuit 66 is made active according to the output signal ofdelay circuit 26, and produces match amplifier activating signal MAE andlatch instructing signal LAT in synchronization with the rising of clocksignal CLK. Discharge control circuit 68 produces discharge instructingsignal DIS in response to the rising of clock signal CLK according tothe output signal of delay circuit 26.

By using control circuit 8 shown in FIG. 24, search data input circuit 4can operate in each search cycle to take in, latch and output the searchdata by flip-flop circuit 94.

In the construction of control circuit 8 shown in FIG. 24, transition ofsearch data SD may be detected, to produce search operation instructionEN according to a signal indicating the detection of the search datatransition.

Frequency divider 90 produces a frequency-divided clock signal CLK bydividing clock signal CLK by the factor of two in the timing chart ofFIG. 22. However, the division ratio of frequency divider 90 and thenumber of delay clock cycle(s) of delay circuit 26 can be set toappropriate values depending on the number of clock cycles in one searchcycle.

According to the sixth embodiment of the invention, as described above,the match line is precharged to the ground voltage level and, in thesearch operation, the match line is supplied with the current of arestricted value that is smaller than the one-bit pull-out current orone-bit miss current but is larger than an all-bit off-leakage current.Therefore, the voltage amplitude of the match line can be reduced, andthe charging current of the match line can be reduced. For search linesSL and /SL, as shown in FIG. 24, flip-flop circuit 94 holds the searchdata. Therefore, when the search data having similar bit patterns aresuccessively supplied, it is possible to reduce the number of searchlines to be charged and discharged in the search data bus, and therebythe charging/discharging currents of the search lines can be reduced.

Seventh Embodiment

FIG. 25 shows a main portion of a content addressable memory accordingto a seventh embodiment of the invention. In the content addressablememory shown in FIG. 25, entry ERY in the memory cell array has the sameconstruction as the entries ERY of the first to sixth embodimentsalready described. Corresponding portions are allotted the samereference numerals, and description thereof is not repeated.

Match amplifier 40 includes discharging transistor 60 for dischargingmatch line ML to the ground voltage level according to dischargeinstructing signal DIS, and a pull-up/sense circuit 100 for producing aninternal search determination result signal MA_ML. Pull-up/sense circuit100 supplies a pulling-up current to match line ML, performs the searchdetermination and produces an internal search determination resultsignal according to a result of the determination.

Pull-up/sense circuit 100 includes P-channel MOS transistors QP71 andQP72 connected in series between the power supply node and an internalnode ND70, an N-channel MOS transistor QN71 connected between internalnode ND70 and corresponding match line ML, and an NOR gate NG1 receivinginternal search determination result signal MA_ML on internal node ND70and precharge instructing signal PRE. NOR gate NG1 applies its outputsignal to MOS transistor QP72.

MOS transistor QP71 receives discharge instructing signal DIS on itsgate, and MOS transistor QN71 receives a bias voltage BIAS_N on itsgate.

Match amplifier 40 further includes latch 16 for latching output signalMA_ML of pull-up/sense circuit 100 according to latch instructing signalLAT. Match amplifier 40 does not have a differential amplifier circuit,and the current consumption thereof can be small.

Bias voltage generating circuit 45 includes P-channel MOS transistorsQP73 and QP74 connected in series between the power supply node and anode ND72, a comparator CMP for comparing the voltage on node ND72 withprecharge voltage VML, an N-channel MOS transistor QN72 connected at afirst conduction node to node ND72, and N-channel MOS transistors QN75and QN76 connected in series between a second conduction node of MOStransistor QN72 and the ground node. N-channel MOS transistor QN72 has agate receiving an output signal of comparator CMP.

MOS transistors QP73 and QP74 have gates coupled to the ground node, andare normally kept conductive. MOS transistors QP73 and QP74 are the samein size (the ratio of the channel width to the channel length) as MOStransistors QP71 and QP72 included in match amplifier 40, respectively.

MOS transistors QN75 and QN76 have gates both coupled to the powersupply node, and are normally kept on. MOS transistors QN75 and QN76have the same sizes as MOS transistors TR3 and TR4 included in unit cellUC, respectively. Therefore, MOS transistors QN75 and QN76 pass one-bitpull-out current In at the maximum.

In the construction of bias voltage generating circuit 45, comparatorCMP compares precharge voltage VML at the intermediate voltage levelwith the voltage of node ND72. When the voltage level of node ND72 ishigher than precharge (intermediate) voltage VML, the output signal ofcomparator CMP becomes high. Accordingly, the conductance of MOStransistor QN72 increases, the current passing from node ND72 to MOStransistors QN75 and QN76 increases, and the voltage level of node ND72lowers. When the voltage level of node ND72 is lower than prechargevoltage VML, the output signal of comparator CMP attains a low level.Accordingly, the conductance of MOS transistor QN72 decreases, and theamount of the current flowing through MOS transistor QN72 is reduced tosuppress the potential lowering of node ND72. Thus, comparator CMPadjusts the conductance of MOS transistor NQ72 such that node ND72 iskept at the level of precharge-voltage-VML. Precharge voltage VML is atthe level not exceeding half the power supply voltage.

In match amplifier 40, MOS transistors QP71 and QP72 are the same insize (ratio of the channel width to the channel length) as MOStransistors QP73 and QP74, respectively. MOS transistors QN71 and QN72have the same size. MOS transistor NQ71 receives output voltage BIAS_Nof comparator CMP on its gate. In match amplifier 40, therefore, signalMA_ML on node ND70 is kept substantially at the level of prechargevoltage VML when corresponding entry ERY is in the one-bit miss(mismatch) state. When the voltage level of match line ML becomes equalto that of node ND70, MOS transistor QN is turned off. Therefore, thevoltage level of match line ML does not exceed the voltage level of nodeND70, and therefore, the voltage level of match line ML is set to thevoltage VML or lower.

MOS transistors QP71 and QP72 are configured to have such sizes thatthey have capabilities of driving a current smaller than the currentflowing through the match line of the entry in the one-bit mismatchstate (i.e., a current smaller a sum (Imiss+(m−1)·Ioff) of the one-bitmiss current and the off-leakage currents of the remaining unit cells).

FIG. 26 is a timing chart representing a search operation of the contentaddressable memory shown in FIG. 25. Referring to FIG. 26, the searchoperation of the content addressable memory shown in FIG. 25 will now bedescribed.

Before time T1, precharge instructing signal PRE is at the L level.Discharge instructing signal DIS is at the H level, and match line ML isprecharged to the ground voltage level. Then, MOS transistor QP71 isoff, and node ND70 is discharged to the ground voltage level as matchline ML is discharged, and is kept at the ground voltage level.

Before time T1, discharge instructing signal DIS is at the H level, anddischarge transistor 60 precharges match line ML to the ground voltagelevel. MOS transistor QP71 is off, and node ND70 is at the groundvoltage level.

At time T1, the search cycle starts. Discharge instructing signal DISattains the L level, discharge transistor 60 is turned off and MOStransistor QP71 is turned on. Precharge instructing signal PRE attainsthe H level, and the output signal of NOR gate NG1 attains the L level:Accordingly, node ND70 is supplied with a current via MOS transistorQP71 and QP72, and the voltage level of node ND70 rises. The currentsupplied via MOS transistors QP71 and QP72 is supplied to match line MLvia MOS transistor QN71. When entry ERY corresponding to match line MLis in the mismatch state, match line ML is discharged via unit cell UCin the miss state. The current supplied via MOS transistors QP71 andQP72 is equal to the one-bit miss current or lower, and the voltagelevel of match line ML does not reach precharge voltage VML and will bedischarged to the level of ground voltage GND at a faster timing.

MOS transistor QN71 receives bias voltage BIAS_N on its gate, and keepsnode ND70 at the level of precharge voltage VML when one-bit pull-outcurrent In passes through corresponding match line ML. When the numberof the unit cells in the miss state in entry ERY is larger than one bit,node ND70 is driven to the voltage level lower than precharge voltageVML. Therefore, node ND70 has the voltage at the level of up toprecharge voltage VML when the corresponding entry is in the mismatchstate.

At time T3, the search determination operation is performed, and latch16 attains the through state according to latch instructing signal LAT,and generates the output signal ML_OUT at the L level (the voltage (upto VML) on node ND70 is at a level sufficiently lower than the inputlogical threshold voltage of latch 16).

In the latch operation, precharge instructing signal PRE is madeinactive at time T3. Therefore, the voltage level of node ND70 is theinput voltage level that is determined as the L level by NOR gate NG1,and NOR gate NG1 produces the output signal at the H level. Accordingly,MOS transistor QP72 is turned off, and the voltage level of node ND70further decreases. Search result indicating signal ML_OUT produced fromlatch 16 is reliably set to the L level.

Precharge instructing signal PRE is driven to the L level at time T3, sothat the voltage level of match line ML lowers. Even if the loweredvoltage level of match line ML does not reach ground voltage GND, matchline ML will be driven to the ground voltage level when latch 16 entersthe latch state and discharge instructing signal DIS attains the H levelin the cycle starting at time T4. Accordingly, signal MA_ML on node ND70is discharged to the ground voltage level (MOS transistor QP72 is off).Thus, the precharging of match line ML and node ND70 is completed.

In a search cycle at time T5, the search operation is executed on nextsearch data. When discharge instructing signal DIS is driven to the Llevel and precharge instructing signal PRE is driven to the H level, MOStransistors QP71 and QP72 are turned on. Accordingly, a current issupplied to node ND70, and is further supplied to match line ML via nodeND 70, so that the voltage level of signal MA_ML on node ND70 rises. MOStransistor QN71 receives bias voltage BIAS_N on its gate, and MOStransistors QN72 and QN71 have the same size. Therefore, MOS transistorQN71 passes the one-bit miss (pull-out) current at the maximum. When thepotential of match line ML in the match state rises, the source anddrain of MOS transistor QN71 attain the same voltage according to thesource follower operation, and MOS transistor QN71 is turned off. Thus,the potential rising of match line ML is suppressed, and match line MLis kept at the level of up to precharge voltage VML.

In this state, MOS transistors QP71 and QP72 are on, and supply thecurrent to node ND70. Therefore, the voltage level of signal MA_ML onnode ND70 will finally rise to the level of power supply voltage VDD.

In a clock cycle starting at time T7, latch instructing signal LATattains the H level so that output signal ML_OUT produced from latch 16attains the H level indicative of the match state.

At time T7, even when precharge instructing signal PRE is at the Llevel, the signal MA_ML on node ND70 is at a sufficiently high voltagelevel, and the output signal of NOR gate NG1 is at the L level. MOStransistor QP72 maintains conductive, and the voltage level of signalMA_ML on node ND70 can be accurately determined to produce search resultindicating signal ML_OUT.

MOS transistor QN71 operates in the source follower mode according toits bias voltage BIAS_N, and is turned off when a gate to source voltagebecomes equal to the threshold voltage. Therefore, even in the statewhere the signal MA_ML on node ND70 rises to the level of power supplyvoltage VDD, MOS transistor QN71 turns non-conductive when the voltagelevel of match line ML rises, to suppress rising of the voltage level ofmatch line ML to precharge voltage VML or higher.

In the construction shown in FIG. 25, the load capacitance of internalnode ND70 of match amplifier 40 is much smaller than the parasiticcapacitance of match line ML. Therefore, the charging current ofinternal node ND70 is smaller in quantity than the charging current thatflows to charging match line ML, and accordingly, the currentconsumption in the search operation can be further reduced.

According to the seventh embodiment of the invention, as describedabove, the match amplifier produces the search result indicating signalby charging the internal node while suppressing the potential rising ofthe match line to the amplitude of the intermediate voltage or lower(VML≦VDD/2) by MOS transistor QN71 that receives bias voltage BIAS_N onits gate. Therefore, the occupation area and power consumption of thematch amplifier can be small, and the current consumption in the searchresult determining operation can be sufficiently small.

For the circuit for generating the control signal in the seventhembodiment, the construction of the control circuit shown in FIG. 24 canbe employed. For the circuit producing precharge instructing signal PRE,such a construction can be employed, by which precharge instructingsignal PRE is to the H level during a period of one clock cycleaccording to search operation instruction EN supplied from the commanddecoder shown in FIG. 24.

Eighth Embodiment

FIG. 27 shows a main portion of a content addressable memory accordingto an eighth embodiment of the invention. The content addressable memoryshown in FIG. 27 differs from the content addressable memory shown inFIG. 25 in the following construction. Match amplifier 40 is providedwith a charge-up circuit 110 for supplying charging electric charges tomatch line ML according to pull-up current supply instructing signalMLPU_n. Charge-up circuit 110 includes P-channel MOS transistors QP81and QP82 connected in series between the power supply node and matchline ML, and a capacitance element CQ2 connected to a connection nodebetween MOS transistors QP81 and QP82.

Other constructions of the content addressable memory shown in FIG. 27are the same as those of the content addressable memory shown in FIG.25. Corresponding portions are allotted the same reference numerals, anddescription thereof is not repeated.

FIG. 28 is a timing chart representing the search operation of thecontent addressable memory shown in FIG. 27. Referring to FIG. 28, thesearch operation of the content addressable memory shown in FIG. 27 willnow be described.

Before time T1, precharge instructing signal PRE is at the L level, anddischarge instructing signal DIS is at the H level. Therefore, matchline ML and signal ML_MA on internal node ND70 are at the ground voltage(GND) level. Latch 16 is in the latch state. The timing chart of FIG. 28shows the case in which search result indicating signal ML_OUT at the Hlevel is produced.

At time T1, the search cycle starts. Discharge instructing signal DISattains the L level, MOS transistor 60 is turned off and MOS transistorQP71 is turned on. At this time, precharge instructing signal PRE isstill at the L level. Therefore, the output signal of NOR gate NG1 is atthe H level, and MOS transistor QP72 is in the off state.

Pull-up current supply instructing signal MLPU_n attains the L level,and charge instructing signal CHA_n attains the H level. Responsively,capacitance element CQ2 is isolated from the power supply node, and thecharged (accumulated) charges in capacitance element CQ2 are transmittedto match line ML and node ND70. By adjusting the capacitance value ofcapacitance element CQ2, the voltage level of match line ML can be setto a voltage level lower than the intermediate voltage level ofprecharge voltage VML.

At this time, pull-up current supply instructing signal MLPU_n rises tothe H level, MOS transistor QP82 is turned off and the pull-up operationeffected by charge-up circuit 110 on match line ML is completed. Thepull-up operation by charge-up circuit 110 is performed utilizingcapacitance element CQ2 and match line ML and signal MA_ML on node ND70are rapidly driven to the predetermined precharge voltage level.

In a search cycle starting at time T1, precharge instructing signal PREattains the H level between times T1 and T2 so that NOR gate NG1generates the output signal at the L level, and MOS transistor QP72 isturned on. Responsively, the pull-up current is supplied to match lineML via MOS transistors QP71, QP72 and QN72.

In the above operation, when entry ERY corresponding to match line ML isin the mismatch state, a current larger than the pull-up currentsupplied by pull-up/sense circuit 100 is discharged to the ground node,and the voltage level of match line ML lowers.

At time T3; precharge instructing signal PRE attains the L level. Atthis time, node ND70 is at the L level, and NOR gate NG1 supplies theoutput signal at the H level, so that MOS transistor QP72 is turned ofNode ND70 is discharged to the ground voltage level from MOS transistorQN71 via unit cell UC in the miss state in entry ERY . In this state,latch instructing signal LAT attains the H level, and latch 16 attainsthe through state. Node ND70 is at the ground voltage level, and latch16 outputs signal ML_OUT at the L level according to internal searchinstructing signal MA_ML.

In a cycle starting at time T4, precharge instructing signal T4 attainsthe H level again. Accordingly, match line ML is precharged to theground voltage level, and node ND70 is discharged to the ground voltagelevel (MOS transistor QP72 is off).

In a period between times T3 and T5, charge instructing signal CHA_nattains the L level, and capacitance element CQ2 is charged toaccumulate electric charges.

In the search cycle starting at time T5, operations similar to theprevious search cycle are performed. Specifically, capacitance elementCQ2 of match line ML charges match line ML and internal node ND70 toraise the voltage levels thereof. When the charge-up operation iscompleted, pull-up current supply instructing signal MLPUn attains the Hlevel, and the charge-up operation is completed.

Then, precharge instructing signal PRE attains the H level, NOR gate NG1generates the output signal at the L level and accordingly, MOStransistor QP72 is turned on. When entry ERY corresponding to match lineML is in the match state, a path of discharging match line ML is notpresent. Therefore, MOS transistors QP71 and QP72 charge the node ND70to raise finally the voltage level of internal search instructing signalMA_ML to the level of power supply voltage VDD. Even when node ND70 ischarged to the power supply voltage level, MOS transistor QN71 preventsthe voltage level of match line ML from exceeding precharge voltage VML.

At time T7, precharge instructing signal PRE attains the L level.However, node ND70 is already at the H level, and the output signal ofNOR gate NG1 is at the L level. Therefore, signal MA_ML on node ND70 iskept at the H level. At time T7, latch instructing signal LAT attainsthe H level, latch 16 enters the through state to produce signal ML_OUTat the H level corresponding to signal MA_ML on node ND70.

During this period, match line ML has no discharging path, and is keptat the level of precharge voltage VML. At time T8, the latch operationof the output signal is performed. Latch instructing signal LAT attainsthe L level, and latch 16 attains the latch state. Also, dischargeinstructing signal DIS attains the L level, match line ML is dischargedto the ground voltage level and node ND70 is likewise discharged to thelevel of ground voltage. Accordingly, the output signal of NOR gate NG1attains the H level, and MOS transistor QP72 is turned off. Thus, nodeND70 and match line ML are reliably discharged to the ground voltagelevel.

Charge-up circuit 110 pulls up match line ML to the predeterminedvoltage level, using capacitance element CQ2, and accordingly, thevoltage levels of match line ML and internal node ND70 can be rapidlychanged, to perform the search operation faster. Capacitance element CQ2is used for the charging, and the voltage VDD on the power supply nodeis not consumed during charging of match line ML. Therefore, occurrenceof power supply noises is suppressed when pulling up the match line. Itis necessary to perform the charging of capacitance element CQ2 merelyover a clock cycle period during the period between times T3 and T5 andbetween times T7 and T8. Therefore, the charging of capacitance elementCQ2 is performed slowly, which can reduce the peak current.

FIG. 29 shows an example of a construction of a circuit for generatingthe control signals in the content addressable memory according to theeighth embodiment of the invention. In FIG. 29, control circuit 8includes command decoder 20 for decoding command CMD in synchronizationwith clock signal CLK, a frequency divider 90 for frequency-dividingclock signal CLK according to the activation of search operationinstruction EN received from command decoder 20, and a search data inputcontrol circuit 92 for producing search latch instructing signal LTEN tothe search data input circuit according to frequency-divided clocksignal BCLK of frequency divider 90 and the search operationinstruction.

Control circuit 8 further includes a charge-up activating circuit 120for producing charge instructing signal CHAn according tofrequency-divided clock signal BCLK of frequency divider 90 and searchoperation instruction EN, a pull-up activating circuit 122 for producingpull-up current supply instructing signal MLPU_n in the form of apredetermined one-shot pulse according to search operation instructionEN and clock signal CLK, and a pull-up activation control circuit 124for producing precharge instructing signal PRE. Pull-up activationcontrol circuit 124 activates (produces) search operation instruction ENin response to falling of pull-up current supply control signal MLPU_nreceived from pull-up activating circuit 122 when search operationinstruction EN is active.

Control circuit 8 further includes delay circuit 26 for delaying searchoperation instruction EN by a period of one clock cycle, a latchactivation control circuit 126 for maintaining latch instructing signalLAT at the H level for a predetermined period according to the pulsesignal from delay circuit 26 and clock signal CLK, and a dischargeactivating circuit 128 for maintaining discharge instructing signal DISat the H level for a predetermined period in response to the falling oflatch instructing signal LAT received from latch activation controlcircuit 126.

Frequency divider 90, search data input control circuit 92, commanddecoder 20 and delay circuit 26 have the same constructions as those inthe control circuit shown in FIG. 24. Charge-up activating circuit 120drives and maintains charge instructing signal CHA_n to the H level fora period of half a clock cycle of frequency-divided clock signal BCLKwhen search operation instruction EN is active.

Pull-up activating circuit 122 has the construction of the one-shotpulse generating circuit, and maintains pull-up current supplyinstructing signal MLPU_n at the L level for a predetermined periodafter the search operation starts. Pull-up activation control circuit124 maintains precharge instructing signal PRE at the H level inresponse to the rising of pull-up instructing signal MLPU_n to the Hlevel until clock signal CLK rises subsequently.

When the output signal of delay circuit 26 is active, latch activationcontrol circuit 126 maintains latch instructing signal LAT at the Hlevel during the H level of clock signal CLK. When latch instructingsignal LAT falls to the L level, discharge activating circuit 128 drivesdischarge instructing signal DIS to the H level, and maintains the Hlevel until clock signal CLK rises subsequently (when delay circuit 26is active).

According to the eighth embodiment of the invention, as described above,the pull-up operation is performed on the match line, using theaccumulated (charged) charges of the capacitance element. In addition tothe effects of the seventh embodiment, the match line can be rapidlydriven to the predetermined voltage level. For pulling up the matchline, the charged electric charges of the capacitance element areutilized, and the production of power supply noises can be suppressedwhen the match lines are pulled up.

Ninth Embodiment

FIG. 30 shows a main portion of a content addressable memory accordingto a ninth embodiment of the invention. The content addressable memoryshown in FIG. 30 differs from the content addressable memory shown inFIG. 27 in constructions of bias voltage generating circuit 45 and latchamplifier 40.

Specifically, bias voltage generating circuit 45 is provided withreplica entry 50 including unit cells having one-bit unit cell in themiss state. This replica entry 50 has the same internal construction asreplica entry 50 in FIG. 21. Replica match line RML arranged for replicaentry 50 is supplied with a current via a P-channel MOS transistor QP93that has a gate and a drain connected to each other. Therefore, acurrent MESS supplied from MOS transistor QP93 has substantially thesame magnitude as the match line current discharged via replica entry 50including the replica unit cells having the one-bit in the miss state,and is equal to a sum of one-bit miss current Imiss in entry ERY and theoff-leakage current flowing through the remaining unit cells in thematch state.

This bias voltage generating circuit further includes a P-channel MOStransistor QP92 forming a current mirror circuit with MOS transistorQP93. MOS transistor QP92 supplies a current to MOS transistor QP74.Other constructions of bias voltage generating circuit 45 are the sameas those of the bias voltage generating circuit shown in FIG. 27.Corresponding portions are allotted the same reference numerals, anddescription thereof will not be repeated.

MOS transistor QP92 is smaller in size than MOS transistor QP93.Therefore, MOS transistor QP92 passes a current Ip2 smaller than acurrent IMISS flowing through MOS transistor QP93, and smaller than thecurrent flowing through the match line in the one-bit mismatch state.

Match amplifier 40 includes a P-channel MOS transistor QP91 receiving agate voltage BIAS_P of MOS transistor QP93 on its gate and supplying acurrent to MOS transistor QP71. MOS transistor QP91 is the same in sizeas MOS transistor QP92. These MOS transistors QP91 and QP92 each form acurrent mirror circuit with MOS transistor QP93, and pass currents Ip1(=IP2) of the same magnitude.

Other constructions of match amplifier 40 are the same as those of thematch amplifier shown in FIG. 27. Corresponding portions are allottedthe same reference numerals, and description thereof will not berepeated.

In the construction of the content addressable memory shown in FIG. 30,bias voltage BIAS_P corresponding to the current equal to or smallerthan the match line current in the one-bit miss state is produced, usingreplica entry 50. Similarly to the construction shown in FIG. 21, evenwith variations in parameters of unit cells UC in entry ERY of thememory cell array, the current flowing through unit cell UC can becorrected. For example, it is assumed that the variations in processparameters cause the current flowing through the P-channel MOStransistor to increase above a normal magnitude, and also cause thecurrent flowing through the N-channel MOS transistor to decrease below anormal magnitude. In this case, the miss current flowing through unitcell UC in the miss state becomes small in entry ERY . In this case,however, current In flowing through the N-channel MOS transistorlikewise decreases in replica entry 50, and accordingly, currents Ip2and Ip1 flowing through MOS transistors QP92 and QP91, respectively,decrease. In entry ERY storing the actual data, therefore, even in thesearch miss, the charging current quantity for match line ML is adjustedaccording to the variations in the discharging current, and the accuratesearch operation can be achieved.

As shown in FIG. 30, the current of the same magnitude as the misscurrent of entry ERY storing one-bit miss data is caused to flow, usingreplica entry 50. Accordingly, current Ip1 flowing through P-channel MOStransistor QP91 can be smaller than match line current MESS (includingan off-leakage current) in the one-bit mismatch state. Thus, the matchline can be charged accurately regardless of variations in processparameters, and the accurate search operation can be achieved.

According to the ninth embodiment of the invention, as described above,the bias voltage generating circuit produces the match line current inthe one-bit miss state, using the replica entry, and adjusts the matchline charging current of the pull-up/sense circuit in the matchamplifier. Therefore, the search operation can be accurately performedregardless of the variations in process parameters. Further, the effectssimilar to those of the eighth embodiment can be achieved.

The effects by this construction using the replica entry can likewise beachieved in the sixth embodiment shown in FIG. 21.

Tenth Embodiment

FIG. 31 shows a main portion of a content addressable memory accordingto a tenth embodiment of the invention. The construction of the contentaddressable memory shown in FIG. 31 differs from the construction of thecontent addressable memory shown in FIG. 30 in the followingconstruction. The content addressable memory in FIG. 31 includes abuffer 130 for converting the level of bias voltage BIAS_N produced bybias voltage generating circuit 45. Buffer 130 converts bias voltageBIAS_N into a bias voltage BIAS_N2 having a slightly boosted level, andvoltage BIAS_N2 produced by the level conversion is applied, as the biasvoltage, to the gate of N-channel MOS transistor QN71 of each matchamplifier 40. Other constructions of the content addressable memoryshown FIG. 31 are the same as those of the content addressable memoryshown in FIG. 30. Corresponding portions are allotted the same referencenumerals, and description thereof is not repeated.

Bias voltage BIAS_N2 is at the voltage level slightly higher by avoltage ΔV than bias voltage BIAS_N provided from bias voltagegenerating circuit 45. Voltage ΔV is 100 mV or lower. When the gatepotential of N-channel MOS transistor QN71 lowers below a desired value,the conductance of MOS transistor QN71 lowers, and the current flowingthrough match line ML is restricted. Node ND70 is charged by a currentcomponent thus restricted, and the voltage level of signal MA_ML on nodeND70 rises. When the search result is the miss, match line ML is driventoward the ground voltage level. However, there is a possibility thatinternal node ND70 in match amplifier 40 is not driven to the groundvoltage level due to the restricted current component described above,but attains the H level to cause a malfunction of erroneouslydetermining the state as the match state. Therefore, bias voltageBIAS_N2 applied to the gate of MOS transistor QP71 is set to the voltagelevel slightly higher than the desired value BIAS_N. Thus, it ispossible to suppress a malfunction that may be caused by lowering ofbias voltage BIAS_N2 toward the ground voltage due to noises, and theaccurate search operation can be achieved.

This embodiment can also achieve the similar effects as the eighth andninth embodiments.

FIG. 32 shows an example of a construction of buffer 130 shown in FIG.31. In FIG. 32, buffer 130 includes P-channel MOS transistors QP101 andQP102 coupled to the power supply node, N-channel MOS transistorsQN101-QN103 connected in series between MOS transistor QP101 and theground node, and N-channel MOS transistors QN104-QN106 connected inseries between MOS transistor QP102 and the ground node.

MOS transistors QP101 and QP102 form a current mirror circuit. P-channelMOS transistor QP101 has a gate and a drain connected to each other, andserves as a master stage of the current mirror circuit. MOS transistorQN101 receives bias voltage BIAS_N on its gate. MOS transistor QN104 hasa gate and a drain connected to each other, and produces level-convertedbias voltage BIAS_N2 at its gate. MOS transistors QN102, QN103, QN105and QN106 have gates coupled to the power supply node. These N-channelMOS transistors QN101-QN106 have the same size. MOS transistor QP101 hasa smaller size than MOS transistor QP102.

In the construction of buffer 130 shown in FIG. 32, MOS transistor QN101operates as a constant current supply according to its bias voltageBIAS_N, and supplies the current to N-channel MOS transistors QN102 andQN103. The current flowing through MOS transistor QN101 is supplied viaMOS transistor QP101. MOS transistor QP102 generates a mirror current ofthe current flowing through MOS transistor QP101. MOS transistor QP102has a larger size than MOS transistor QP101. Therefore, MOS transistorQP102 causes a flow of a larger current than MOS transistor QP101 does.

The current is discharged from MOS transistor QP102 to the ground nodevia MOS transistors QN104-QN106. MOS transistor QN104 has a gate and adrain connected together, and produces on its gate a signal subjected tothe current/voltage conversion, i.e., bias voltage BIAS_N2. MOStransistor QN104 passes a larger current than MOS transistor QN101. MOStransistors QN101 and QN104 have the same size. Therefore, MOStransistor QN104 produces bias voltage BIAS_N2 at a higher level thanbias voltage BIAS_N.

In buffer 130, MOS-transistors QN101-QN106 are formed in the samemanufacturing steps as the MOS transistors employed for discharging thematch line of the replica entry. Thus, it is possible to compensate forvariations in transistor characteristics due to process variations, andbias voltage BIAS_N2 can be set to a voltage level of a desired value.

According to the tenth embodiment of the invention, as described above,the buffer raises the level of the bias voltage to be supplied to thegate of the MOS transistor that supplies the current to the match line.Therefore, the accurate search operation can be performed even whennoises occur towards the ground direction on the bias voltage.

The effects similar to those of the ninth embodiment can be alsoachieved.

Eleventh Embodiment

FIG. 33 shows a main portion of the content addressable memory accordingto the eleventh embodiment of the invention. The content addressablememory shown in FIG. 33 differs from the content addressable memoryshown in FIG. 31 in the following construction. Bias voltage generatingcircuit 45 includes a constant current circuit 140 that produces a biasvoltage BIAS_P0 to be applied to the gate of MOS transistor QP92. Thereis also provided a current converter circuit 135 that performs the levelconversion on bias voltage BIAS_P0 to apply the level-converted voltageto the gate of MOS transistor QP91 in match amplifier 40. Matchamplifier 40 and entry ERY in the memory cell array shown in FIG. 33have the same constructions as those shown in FIG. 33. Otherconstructions of bias voltage generating circuit 45 are the same asthose shown in FIG. 31. Corresponding portions are allotted the samereference numerals, and description thereof is not repeated.

Constant current circuit 140 includes P-channel MOS transistors QP93 andTN71 connected in series between the power supply node and an internalnode ND73, N-channel MOS transistors TN75 and TN76 connected in seriesbetween internal node ND73 and the ground node, and a comparing circuitCMPA for comparing the voltage on internal node ND73 and intermediatevoltage VML. Comparing circuit CMPA adjusts the gate potential of MOStransistor TN71 according to a result of the comparison.

MOS transistor QP93 has a gate and a drain connected to each other,forms a current mirror circuit with MOS transistor QP92. 3 MOStransistors QP92 and QP93 have the same size, and pass the currents ofthe same magnitude.

MOS transistors TN75 and TN76 have gates coupled to the power supplynode, and are always on. MOS transistors TN75 and TN76 have the samesizes as transistors TR3 and TR4 or transistors TR1 and TR2 in unit cellUC, respectively, and pass one-bit pull-out current In (Imiss).

Comparing circuit CMPA has a positive input receiving precharge voltageVML and a negative input coupled to internal node ND73. Through thefeedback control of comparing circuit CMPA and MOS transistor TN71, thevoltage level of internal node ND73 is set equal to the level ofintermediate voltage (precharge voltage) VML. Therefore, the voltage ofnode ND73 accurately is held at the level of precharge voltage VML evenwhen variations occur in power supply voltage VDD. Thus, one-bitpull-out current In (=Imiss) can be accurately produced without aninfluence of the variations in power supply voltage.

In constant current circuit 140, therefore, MOS transistor QP93 passesthe current of the same magnitude as the current flowing when theone-bit unit cell in the miss state passes one-bit pull-out current In(Imiss) through match line ML. Therefore, bias voltage BIAS_P0 producedby MOS transistor QP93 is at the voltage level corresponding to theone-bit pull-out current. MOS transistor QP92 has the same size as MOStransistor QP93, and current Ip2 is the same in magnitude as one-bitpull-out current In (=Imiss).

Current converter circuit 135 performs the level conversion on biasvoltage BIAS_P0 to raise its voltage level slightly. Thereby, matchamplifier 40 can pass, through MOS transistor QP91, current Ip1 that issmaller than one-bit pull-out current In and is larger than a totalcurrent IOFF of the off-leakage current of all the bits.

This bias voltage generating circuit 45 holds the internal nodeconnected to the replica search transistors at the level of prechargevoltage VML of the match line. Thus, it is possible in the match lineprecharge state to produce stably the current of a magnitude nearlyequal to that of the current discharged by the one-bit unit cell in themiss state so that the amplitude of the match line can be restrictedmore accurately to reduce the current consumption.

Construction 1 of Current Converter Circuit

FIG. 34 shows an example of a construction of current converter circuit135 shown in FIG. 33. In FIG. 34, current converter circuit 135 includesa P-channel MOS transistor TP100 that is coupled to the power supplynode and receives bias voltage BIAS_P0 on its gate, and an N-channel MOStransistor TN100 receiving a current from MOS transistor TP100.

Constant current circuit (140) produces bias voltage BIAS_P0. MOStransistor TN100 has a gate and a drain connected to each other.

Current converter circuit 135 further includes an N-channel MOStransistor TN101 and a P-channel MOS transistor TP101 supplying acurrent from the power supply node to MOS transistor TN101. MOStransistor TN101 form a current mirror circuit with MOS transistorTN100. MOS transistor TP101 has a gate and a drain connected to eachother.

MOS transistor TP101 produces bias voltage BIAS_P on its gate, andapplies bias voltage BIAS_P to the gate of MOS transistor QP91 of matchamplifier 40.

In bias voltage generating circuit 45, MOS transistors QP92 and QP93 arethe same in size (ratio of channel length L over channel width W, W/L),and pass current Imiss of the same magnitude. MOS transistor TP100 issmaller in size than MOS transistors QP92 and QP93. Therefore, currentIp flowing through MOS transistor TP is smaller than current Imissdriven by the replica search transistor.

MOS transistors TN100 and TN101 are the same in size, and pass thecurrents of the same magnitude. MOS transistor TN101 is supplied with acurrent from MOS transistor TP101, and current Ip1 flows through MOStransistor TP101. MOS transistors TP101 and QP91 are the same in size,and pass the currents of the same magnitude. Thus, match amplifier 40can supply, as the pull-up current, the current smaller than the one-bitpull-out current to the match line.

Construction 2 of Current Converter Circuit

FIG. 35 shows a construction of a modification of the current convertercircuit shown in FIG. 33. In the construction shown in FIG. 35, each ofMOS transistors QP92 and QP93 is formed of a parallel connection of Kunit P-channel MOS transistors UPT, and passes the one-bit pull-outcurrent. MOS transistor TP101 in current converter circuit 135 is formedof a parallel connection of unit P-channel MOS transistors of J innumber smaller than K (J<K). Other constructions of current convertercircuit 135 are the same as those shown in FIG. 34. Correspondingportions are allotted the same reference numerals, and descriptionthereof is not repeated.

Unit transistor UPT has a channel width and a channel length each set toa unit value. Therefore, a total channel width of MOS transistor TP101is smaller than a total channel width of each of MOS transistors QP93and QP92. Accordingly, current Ip1 flowing through MOS transistor TP101can be smaller than the current flowing through each of MOS transistorsQP92 and QP93. By merely adjusting the number of unit transistors, arelationship (a mirror ratio) between current Imiss (=In) and currentIp1 can be set. Thus, the match line pull-up current can be set to adesired value without any influence of variations in manufacturingparameters.

Construction of Buffer

FIG. 36 shows a construction of buffer 130 shown in FIG. 33. In theconstruction shown in FIG. 33, buffer 130 may be configured using theconstruction shown in FIG. 32. Alternatively, the buffer shown in FIG.36 can be applied to the buffer shown in FIG. 33.

Referring to FIG. 36, buffer 130 includes a P-channel MOS transistorTP102 having a gate and a drain connected to each other, and is suppliedwith a current from the power supply node, and an N-channel MOStransistor TN102 receiving a bias voltage BIAS_N on its gate andsupplied with a current from MOS transistor TP102. Bias voltagegenerating circuit (45) supplies bias voltage BIAS_N.

Buffer 130 includes a P-channel MOS transistor TP103 supplied with acurrent from the power supply node, an N-channel MOS transistor TN103supplied with a current from MOS transistor TP103, and an N-channel MOStransistor TN104 connected between a ground and a common source of MOStransistors TN102 and TN103.

P-channel MOS transistor TP103 form a current mirror circuit with MOStransistor TP102. MOS transistor TN103 has a gate and a drain connectedto each other. MOS transistor TN104 receives an activation signal ACT onits gate.

MOS transistor TP102 is smaller in size than MOS transistor TP103. MOStransistor TP103 passes a larger current (a mirror ratio being largerthan unity). MOS transistors TN102 and TN103 are the same in size. MOStransistor TN104 controls the activation of buffer 130 according toactivation signal ACT. A quantity of current supplied from MOStransistor TN103 to MOS transistor TN103 is larger than an quantity of acurrent supplied from TP102 to MOS transistor TN102. Therefore, thepotential of the gate of MOS transistor TN103 is higher than the gatepotential of MOS transistor TN102. Accordingly, bias voltage BIAS_N2 canbe higher than bias voltage BIAS_N by 100 mV or so.

Sources of MOS transistors TN102 and TN103 are commonly connected totransistor TN104. Therefore, MOS transistors TN103 and TN104 have thesame source potentials, and bias voltage BIAS_N2 can be accuratelyproduced according to a difference between the respective currentamounts flowing through transistors TN102 and TN103.

The effect of buffer 135 is the same as that of the tenth embodimentshown in FIG. 31, and the noise margin for the lowering of the biasvoltage can be increased.

In the buffer 135 shown in FIG. 36, in order to adjust the mirror ratiothrough adjustment of the sizes of MOS transistors TP102 and TP103, theparallel connections of the unit transistors may be employed as shown inFIG. 35, with the number of the unit transistors in each parallelconnection adjusted for adjusting the mirror ratio.

[Modification]

FIG. 37 shows a construction of a modification of the eleventhembodiment of the invention. The content addressable memory shown inFIG. 37 differs from the content addressable memory shown in FIG. 33 inthe following construction. In a constant current circuit 140 shown inFIG. 37, a resistance element ZR is employed instead of MOS transistorsTN75 and TN76. This resistance element ZR has the resistance of the samevalue as the combined on-resistance of MOS transistors TR1 and TR2 ortransistors TR3 and TR4 in the discharging path of the unit cell in themiss state when match line ML is at the voltage VML.

Other constructions shown in FIG. 37 are the same as those of thecontent addressable memory in FIG. 33. Corresponding portions areallotted the same reference numerals, and description thereof is notrepeated.

Resistance element ZR has a resistance value equal to a combinedon-resistance value of the series of search MOS transistors in the unitcell, and thus has a low resistance value. Therefore, even whenresistance element ZR is formed of a metal interconnection line, theoccupation area thereof is small, and increase in circuit occupationarea can be suppressed. By employing resistance element ZR implementedby the metal interconnection line or the like, it is possible toimplement the resistance element being small in variations of processparameter, and accordingly a constant current of a desired magnitude canbe stably produced.

According to the eleventh embodiment of the invention, as describedabove, the constant current circuit is used to pass the one-bit pull-outcurrent under the state when the match line voltage is at the level ofthe precharge voltage, and the mirror current of the passing current isused to adjust the quantity of the pull-up current supplied to the matchline in the match amplifier. Therefore, the match line can accuratelyand stably pass the current equal to or smaller than the current flowingin the one-bit miss state, or one-bit miss current.

Effects similar to those of the tenth embodiment can be also achieved.

Twelfth Embodiment

FIG. 38 shows a construction of a content addressable memory accordingto a twelfth embodiment of the invention. In the content addressablememory shown in FIG. 38, match amplifier 150 arranged corresponding toeach respective match line ML is different in construction from thematch amplifiers in the embodiments described previously. Each matchamplifier arranged corresponding to a respective match line ML has thesame construction as the others, and FIG. 38 representatively shows theconstruction of match amplifier 150 arranged for one match line.

According to the search result in the last search cycle, match amplifier150 according to the twelfth embodiment charges corresponding match lineML and in addition, sets the potential level determination reference forthis match line.

Specifically, match amplifier 150 includes a sense circuit 152 forsensing the potential level of match line ML, a latch circuit 154 forlatching the output signal of sense circuit 152 according to a searchinstructing signal SRCH, and a charge circuit 156 for selectivelysupplying a charge current I_charge to corresponding match line ML inthe search operation. Charge circuit 156 controls the supply of chargingcurrent according to output signal ML_OUT of latch circuit 154.

Latch circuit 154 receives search instructing signal SRCH on its clockinput CK. When search instructing signal SRCH is at the H level, latchcircuit 154 attains the through state, to output a signal received atits D input from a Q output.

Sense circuit 152 includes a P-channel MOS transistor QP113 connectedbetween the power supply node and an internal signal line MALI andhaving a gate coupled to corresponding to match line ML. Sense circuit152 also includes an N-channel MOS transistor QN112 connected betweenthe internal signal line and the ground node and having a gate coupledto corresponding match line ML.

Sense circuit 152 further includes an inverter 163 receiving the signalMALI from MOS transistors QP113 an QN112, to produces an output signalto input D of D-latch circuit 154, and an inverter 161 receiving theoutput signal from output Q of D-latch circuit 154. Sense circuit 152further includes a D-latch circuit 162 receiving the output signal ofinverter 161 at its input D, and N-channel MOS transistors QN113 andQN114 connected in series between an internal signal line MALI and theground node.

D-latch circuit 162 attains the latch state when search instructingsignal SRCH applied to clock input CK is at the H level, and attains thethrough state when search instructing signal SRCH is at the L level.

MOS transistor QN113 has a gate coupled to match line ML. MOS transistorQN114 receives on its gate an output signal DVTH from output Q ofD-latch circuit 162.

In sense circuit 152, MOS transistors QP113 and QN112-QN114 form aninverter buffer for sensing the potential of corresponding match lineML, and the input logical threshold voltage of this inverter buffer iscorrected according to output signal DVTH of D-type latch circuit 162.When MOS transistor QN114 is conductive (signal DVTH is at the H level),the input logical threshold voltage of this inverter buffer is low. WhenMOS transistor QN114 is non-conductive, the input logical thresholdvoltage of the inverter buffer is high. Therefore, the potentialdetermination reference for match line ML is adjusted according to aresult of the search in the immediately preceding cycle.

Charge circuit 156 includes P-channel MOS transistors QP110 and QP111each coupled to the power supply node. MOS transistors QP110 and QP111form a current mirror circuit. Charge circuit 156 further includesN-channel MOS transistors NQ110 and NQ111 connected between MO Stransistor QP110 and the ground node, an EXNOR circuit 160 receiving theoutput signal of D-type latch circuit 154 and search instructing signalSRCH, and a P-channel MOS transistor QP112 connected between MOStransistor QP111 and match line ML. MOS transistor QP112 receives on itsgate an output signal /CHRG of EXNOR circuit 160.

MOS transistors QN110 and QN111 have gates coupled to the power supplynode, and are normally on. MOS transistors QN110 and QN111 are the samein size as transistors TR3 and TR4 or transistors TR1 and TR2 for matchline discharging included in unit cell UC, respectively, and pass acurrent of the same magnitude as one-bit miss current Imiss flowingthrough one unit cell UC in the miss state (assuming that an off-leakagecurrent in the unit cell in the mismatch state is neglected (In=Imiss)).

MOS transistor QP110 has a gate and a drain connected to each other, andoperates as a master stage of the current mirror circuit. MOS transistorQP110 is larger in size (a ratio of a channel width to a channel length)than MOS transistor QP111 (a transconductance gm(QP111) of MOStransistor QP111 is smaller than a transconductance gm(QP110) of MOStransistor QP110). Therefore, when MOS transistor QP112 is on, chargingcurrent I_charge supplied to corresponding match line ML is set to asmaller value than one-bit miss current Imiss (although it is set to alarger value than a total of off-leakage currents IOFF of all the bits).

In the search operation, charge circuit 156 likewise operates to turn onselectively P-channel MOS transistor QP112 to charge match line MLaccording to search result (ML_OUT) in the last search cycle.

FIG. 39 shows, in a table form, operation logic of charge circuit 156shown in FIG. 38. In the search operation, search instructing signalSRCH attains the H level. When the search result in the last searchcycle indicates the match state and the signal ML_OUT is at the H level,EXNOR circuit 160 produces the output signal at the H level.Accordingly, MOS transistor QP112 is turned off to stop the charging ofmatch line ML (current I_charge is off in the table). Conversely, whenthe search result in the last search cycle indicates the mismatch stateand the signal ML_OUT is at the L level, EXNOR circuit 160 produces theoutput signal at the L level. Accordingly, MOS transistor QP112 isturned on to supply current I_charge to corresponding match line ML(current I_charge is on in the table).

In the standby state, search instructing signal SRCH is set to the Llevel. When the signal ML_OUT in the last search cycle is at the H levelindicative of the match state, EXNOR circuit 160 produces output signal/CHRG at the L level. Accordingly, MOS transistor QP112 is turned on tosupply the current to match line ML. When the mismatch state is detectedin the last search cycle, the output signal /CHRG of EXNOR circuit 160is at the H level. In this state, therefore, MOS transistor QP112 is inthe off state or non-conductive state.

In the search operation, therefore, when the determination result in thelast or immediately preceding search cycle indicates the mismatch stateand there is a possibility that the corresponding match line is chargedin the currently performed search cycle, the current is supplied tocorresponding match line ML. During the standby state, the match linethat exhibits the match state in the last search cycle is charged to b ekept at the H level. The match line that exhibits the mismatch state isnot charged during the standby, and corresponding match line ML is keptat the low level of ground voltage attained in the search operation.This is done for preparing for the state transition of the match line inthe next search cycle.

FIG. 40 is a timing chart representing an operation of match amplifier150 shown in FIG. 38. Referring to FIG. 40, the operation of the contentaddressable memory shown in FIG. 38 will now be described.

The search data on search lines SL and /SL is exchanged for each clockcycle. It is now assumed that match line ML is kept at the groundvoltage level. When the search cycle starts at a time T10 and searchinstructing signal SRCH attains the H level, EXNOR circuit 160 producesthe output signal at the L level as shown in the operation logic tableof FIG. 39, and MOS transistor QP112 is turned on. In the search cyclestarting at time T10, when the search data matches the stored data, adischarge path is not present for the corresponding match line ML, andthe voltage level of match line ML rises. Search instructing signal SRCHis at the H level, and D-type latch circuit 162 is in the latch state.According to search result indicating signal ML_OUT in the last cycle,the signal DVTH applied from D-type latch circuit 162 is at the H level.Therefore, an input logic threshold voltage VTH of sense circuit 152 isat a low voltage level. When match line ML is charged to have thevoltage level raised from the ground voltage level, the signaltransmitted from sense circuit 152 to internal signal line MALI attainsthe H level at a faster timing in accordance with input logic thresholdvoltage VTH at a low voltage level.

When search instructing signal SRCH is at the H level, D-type latchcircuit 154 enters the through state to take in and output the signal oninternal signal line MALI provided via inverter 163. In this state,therefore, output signal ML_OUT of D-type latch circuit 154 changesaccording to the signal on internal signal line MALI when searchinstructing signal SRCH is at the H level. When search instructingsignal SRCH is the L level, D-type latch circuit 154 enters the latchstate to latch the taken signal and hold the search result.

During the search period of search instructing signal SRCH being at theH level, therefore, when the voltage level of match line ML rises andthe voltage level of internal signal line MALI lowers, the signal ML_OUTapplied from D-type latch circuit 154 attains the H level according tothe output signal of inverter 163. Therefore, the signal /CHRG appliedfrom EXNOR circuit 160 attains the H level, and the charging of matchline ML temporarily stops.

When the search period expires and search instructing signal SRCHattains the L level, EXNOR circuit 160 produces the output signal /CHRGat the L level again, and match line ML is charged. In this state,D-type latch circuit 154 is in the latch state.

Match line ML thus charged attains the power supply voltage level.D-type latch circuit 154 is in the latch state, and its output signalML_OUT does not change even when match line ML is charged.

At this time, output signal DVTH of D-type latch circuit 162 is at the Hlevel. Therefore, input logic threshold voltage VTH of sense circuit 152is low.

In the search cycle starting at a time T11, the search operation for thesearch data is performed again. In this search cycle, when the result ofthe search in the last cycle indicates in the match state, output signalDVTH of D-type latch circuit 162 attains the L level according to therising of search instructing signal SRCH, and MOS transistor QN114 isturned off. Accordingly, input logic threshold voltage VTH of sensecircuit 152 is set to a high voltage level.

In the search cycle, when search instructing signal SRCH attains the Hlevel, output signal /CHRG of EXNOR circuit 160 attains the H level.Thereby, MOS transistor QP112 is turned off, and the operation ofcharging match line ML stops. According to the search result, match lineML is in the match state, and is not discharged, and the signal oninternal signal line MALI keeps the L level.

When the search period expires and search instructing signal SRCHattains the L level, D-type latch circuit 154 enters the latch state tohold its output signal ML_OUT at the H level. In charge circuit 156,EXNOR circuit 160 produces output signal /CHRG at the L level.Accordingly, MOS transistor QP112 charges match line ML, and match lineML keeps the power supply voltage level. In entry ERY , unit cells UC,e.g., of 72-288 bits are connected, and off-leakage currents flowthrough these unit cells in the match state when match line ML is in thematch state. When the number of the search data bits is large, the totaloff-leakage current becomes innegligibly large. This supply of chargecurrent I_charge suppresses the voltage lowering of the match line,which may be caused by the off-leakage currents of the entry in thematch state.

In the search cycle starting at a time T12, when the search data doesnot match the stored data of the entry (in the mismatch state), andsearch instructing signal SRCH attains the H level, match line ML isdischarged through unit cell UC in the miss state, and the voltage levelof the match line lowers. Accordingly, the signal on internal signalline MALI rises to the H level, and output signal ML_OUT of D-type latchcircuit 154 lowers to the L level. In charge circuit 156, the outputsignal /CHRG of EXNOR circuit 160 attains the L level, and accordingly,MOS transistor QP112 is turned on. Thereby, current I_charge istemporarily supplied to match line ML. However, current I_chargesupplied via MOS transistor QP112 is smaller than discharging currentImiss of match line ML in the mismatch state, and match line ML holdsthe L level. At this time, input logic threshold voltage VTH of sensecircuit 152 is at a high voltage level. Therefore, the voltage loweringof match line ML is sensed at a faster timing, and the voltage level ofinternal signal line MALI lowers. D-type latch circuit 154 is in thethrough state, and output signal ML_OUT thereof changes according to thesignal on internal signal line MALI. Even when match line ML has notbeen charged, the discharging current larger than the charging currentis passed, so that the voltage level of the match line lowers fast.

When search instructing signal SRCH attains the L level, D-type latchcircuit 154 enters the latch state, and the output signal ML_OUT thereofis kept at the L level according to the search result of the currentsearch cycle. Accordingly, output signal /CHRG of EXNOR circuit 160 iskept at the H level. Therefore, match line ML holds the discharged stateat the ground voltage level.

In a cycle starting at a time T13, when search instructing signal SRCHrises to the H level, output signal /CHRG of EXNOR circuit 160 attainsthe L level, and charge current I_charge is supplied to match line ML.At this time, D-type latch circuit 162 produces output signal DVTH atthe H level to set input logic threshold voltage VTH of sense circuit152 to the low voltage level. Charging current I_charge supplied fromcharge circuit 156 is smaller than the current Imiss (=In) flowingthrough the one-bit of unit cell in the miss state. Therefore, thevoltage level of match line ML is substantially kept at the groundvoltage level, and the signal on internal signal line MALI is also keptat the L level. In this state, match line ML and search resultindicating signal ML_OUT keep the same state as that in the last searchcycle.

As shown within a broken line circle, when the state changes from themismatch state to the match state or vice versa, match line ML changesits voltage level, and the charging or discharging is performed. Thecharging or discharging of search lines SL and /SL is performed in thecycle where the bit of the search data changes. Therefore, the currentconsumption can be reduced by reducing the number of times of thevoltage transition on these search lines and the match lines.

FIG. 41 shows, in a table form, the charge consumption per search cycle.For a comparison, FIG. 41 also shows the charge consumption in the caseswhen the match line and the search lines are precharged to the level ofpower supply voltage VDD and when precharged to ground voltage GND.

In FIG. 41, M and N represent the numbers of the match lines and thesearch lines, respectively. Cm and Cs represent a capacitance per matchline and a capacitance per search line, respectively.

When the voltage transition occurs on only one match line and the VDDprecharge scheme is employed, the match line changes from the level ofpower supply voltage VDD to the ground voltage level. Therefore, thecharge consumption in this case is equal to (M−1)·Cm·V, where Vrepresents a voltage difference between power supply voltage VDD andground voltage GND.

When the match line is precharged to ground voltage GND, the match linecausing the voltage transition changes from the ground voltage level tothe power supply voltage level. The other match lines keep the level ofground voltage GND. Therefore, the charge consumption per search cycleis Cm·V. In the embodiment, when the mismatch and the match occuralternately on one match line, the charges of (½)·Cm·V are consumed persearch cycle. However, when the match or mismatch state continueswithout causing the state transition, the charge consumption is zero(the voltage level of the match line does not change).

When the number of the match lines causing the voltage transition are(M/2), the charge consumption per search cycle is (M/2)·Cm·V in eitherthe VDD precharge scheme or the ground voltage (GND) precharge scheme.In the eleventh embodiment, when the match and miss alternately occur,the charge consumption per search cycle is (M/4)·Cm·V. When the match ormiss continue, the charge consumption is zero.

Since the search line is charged/discharged in each search cycleaccording to either the VDD precharge scheme or the GND prechargescheme, charges of N·Cs·V are consumed. In this embodiment, when thematch and mismatch of the search data alternately occur, the chargeconsumption on the search line in question is (N/2)·Cs·V. When thesearch data are continuously the same, the charge consumption is zero.

In the twelfth embodiment of the invention, as described above, chargeconsumption on the match line and the search line is zero in the searchcycle when the match state or mismatch state continue.

In the search operation, the match line(s) attaining the match state areusually fewer than the match lines in the mismatch state. In general,the state transition does not occur on many match lines in the searchoperation. Therefore, the current consumption can be reduced by thetwelfth embodiment in which the match lines are selectivelycharged/discharged based on the search result in the last cycle.

According to the table in FIG. 41, the voltage amplitudes of the matchline and the search line are equal to power supply voltage V (=VDD) inthe twelfth embodiment. However, match line ML in the match state may beat the intermediate voltage level of VDD/2 or lower, as is done in theconstructions of the sixth to eleventh embodiments.

[Modification]

FIG. 42 shows a main portion of a content addressable memory accordingto the twelfth embodiment of the invention. FIG. 42 shows a constructionof a sense circuit of one match amplifier. The match amplifier includescharge circuit 156, similarly to the construction shown in FIG. 38.

In FIG. 42, sense circuit 152 includes a differential amplifier circuit190 for comparing the voltage on match line ML with a reference voltageVref Differential amplifier circuit 190 is of a current mirror type, andhas an operation current defined according to a bias voltage BIAS. Theoutput signal of differential amplifier circuit 190 is applied to aninput D of D-type latch circuit 16. D-type latch circuit 16 takes in thesignal at the input in synchronization with the rising of searchinstructing signal SRCH, and outputs a search result indicating signalML_OUT. D-type latch circuit 16 attains the latch state when searchinstructing signal SRCH applied to clock input CK attains the L level.

Sense circuit 152 further includes a transmission gate 188 receiving areference voltage VrefH from a high reference voltage generating circuit182 and a transmission gate 189 receiving a reference voltage VrefL froma low reference voltage generating circuit 184.

Reference voltage generating circuits 182 and 184 are arranged commonlyto the match amplifiers arranged for the respective match line.Reference voltage VrefH is higher than reference voltage VrefL.

Transmission gates 188 and 189 are selectively turned on according tothe output signals of a D-type latch circuit 180 and an inverter 186.Specifically, when an output signal ML_OD of D-type latch circuit 180 isat the H level, transmission gate 188 is conductive, and transmissiongate 189 is non-conductive. When output signal ML_OD of D-type latchcircuit 180 is at the L level, transmission gate 189 is conductive, andtransmission gate 188 is non-conductive. The reference voltages selectedby transmission gates 188 and 189 are used as a reference fordetermining the match line voltage level in differential amplifiercircuit 190.

Sense circuit 152 further includes D-type latch circuit 180 that takesin search result indicating signal ML_OUT applied from D-type latchcircuit 16 in synchronization with the falling of search instructingsignal SRCH, and inverter 186 inverting the output signal of D-typelatch circuit 180. D-type latch circuit 180 enters the latch state whensearch instructing signal SRCH attains the H level. Output signal ML_ODof D-type latch circuit 180 is applied to EXNOR circuit 160 in chargecircuit 156 shown in FIG. 38.

FIG. 43 is a timing chart representing an operation of the matchamplifier shown in FIG. 42. Referring to FIG. 43, description will nowbe given on the operation of the match amplifier shown in FIG. 42 andparticularly the operation of sense circuit 152.

When the search result indicates the mismatch state in a cycle before acycle starting at time T10, D-type latch circuit 180 produces outputsignal ML_OD at the L level. In this state, transmission gate 189 isconductive, and transmits, as a reference voltage of differentialamplifier circuit 190, reference voltage VrefL received from lowreference voltage generating circuit 184. At this time, output signal/CHRG of the EXNOR circuit in the pull-up current supply circuit (notshown in FIG. 42) is at the L level, and the current is supplied to thematch line.

In the cycle starting from time T10, when the search data matches thestored data, the voltage level of match line ML rises. When the voltagelevel of match line ML exceeds reference voltage VrefL, output signalMALI of differential amplifier circuit 190 attains the H level, andoutput signal ML_OUT of D-type latch circuit 16 attains the H level.When search instructing signal SRCH falls to the L level insynchronization with clock signal CLK in this cycle, D-type latchcircuit 180 enters the through state to produce output signal ML_OD atthe H level. For holding the voltage level of the match line in thematch state, the signal /CHRG is kept at the L level. In response to therising of output signal ML_OD of D-type latch circuit 180, transmissiongate 189 is turned off, and transmission gate 188 is turned on. Thereby,reference voltage VrefH supplied from high reference voltage generatingcircuit 182 is supplied as the reference voltage to differentialamplifier circuit 190.

In the cycle starting at time T11, when match line ML is in the matchstate, the match amplifier does not change its state.

In the cycle starting at time T12, clock signal CLK and searchinstructing signal SRCH rise to the H level and the search operation isperformed. In this cycle, when the search result indicates the mismatchstate, the voltage level of match line ML lowers. At this time,reference voltage Vref of differential amplifier circuit 190 is at thehigh voltage level (VrefH). Therefore, output signal MALI ofdifferential amplifier circuit 190 attains the L level at a fastertiming after the potential of match line ML lowers, and search resultindicating signal ML_OUT applied from D-type latch circuit 16 attainsthe definite state at a faster timing.

In this cycle, when search instructing signal SRCH attains the L levelin synchronization with the falling of clock signal CLK, D-type latchcircuit 180 produces output signal ML_OD at the L level. Accordingly,transmission gate 189 is tuned on, and transmission gate 189 is turnedoff so that low reference voltage VrefL is set as reference voltage Vrefof differential amplifier circuit 190. In the cycle starting at timeT12, the result of the search performed in the last search cycleindicates the match state, and the signal /CHRG is held at the H level.Therefore, the pull-up current is not supplied to the match line.

In the cycle starting at time T13, the result of the search performed inthe last search cycle indicates the mismatch state, and the pull-upcurrent is supplied at the start of the search operation in this cycle.However, the search result indicates the mismatch in this cycle, and thevoltage level of match line ML is held at the L level. Accordingly,search result indicating signal ML_OUT and output signal ML_OD of D-typelatch circuit 180 are kept at the L level.

Reference voltage Vref of differential amplifier circuit 190 is setaccording to the search result of the last search cycle, whereby thereference voltage of the differential amplifier circuit can be set tothe voltage level close to the voltage level of the match line.Accordingly, the search result can be determined at a faster timing, andthe search operation speed can be increased.

According to the twelfth embodiment of the invention, as describedabove, the charging of the match line is selectively performed accordingto the result of the last search cycle, so that the current consumptioncan be further reduced. The input logic threshold voltage of the sensecircuit is adjusted according to the search result in the last searchcycle. Therefore, the input logic threshold voltage can be set to thevoltage level close to the voltage level of the match line, and thevoltage changes on the match line can be sensed at a faster timing.

Thirteenth Embodiment

FIG. 44 shows a main portion of a content addressable memory accordingto a thirteenth embodiment of the invention. In FIG. 44, match amplifier150 includes sense circuit 152, D-type latch circuit 154 and chargecircuit 156 similarly to the twelfth embodiment. Charge circuit 156differs from charge circuit 156 shown in FIG. 33 in the followingconstruction. P-channel MOS transistors QP120 and QP112 are arranged inseries between the power supply node and match line ML. MOS transistorQP120 receives a bias voltage PBIAS on its gate. MOS transistor QP112receives an output signal /CHRG of EXNOR circuit 160 on its gate.

For producing bias voltage PBIAS, a replica entry 200 and a bias voltagegenerating circuit 210 are provided. Replica entry 200 includes replicaunit cells UCh each having a discharging path of the same constructionas unit cell UC in the match state of entry ERY in the memory cellarray.

Replica unit cells UCh includes N-channel MOS transistors QN121 andQN122 connected in series between a replica match line RMLb and theground node, and N-channel MOS transistors QN123 and QN124 connected inseries between replica match line RMLb and the ground node. These MOStransistors QN121-QN124 are the same in size (ratio of channel width Wto channel length L) as transistors TR1-TR3 in unit cell UC. Each of MOStransistors QN121-QN124 is turned off when it receives the groundvoltage on its gate. Therefore, ofd leakage current 2·I_off (=Ioff)flows through replica unit cell UCh. This magnitude of the off-leakagecurrent is the same as that of the current flowing through unit cell UCin the match state. Off-leakage current I_off represents an off-leakagecurrent flowing through one discharging path in the unit cell.

Bias voltage generating circuit 210 includes a P-channel MOS transistorQP121 supplying a current to replica match line RMLb. MOS transistorQP121 has a gate and a drain connected to each other. Bias voltagegenerating circuit 210 further includes P-channel MOS transistors QP122and QP124. MOS transistor QP122 forms a current mirror circuit with MOStransistor QP121. MOS transistor QP124 is coupled to the power supplynode, and has a gate and a drain connected to each other.

Bias voltage generating circuit 210 further includes N-channel MOStransistors QN125 and QN126 connected in series between MOS transistorQP124 and the ground node, and a P-channel MOS transistor QP123 forminga current mirror circuit with MOS transistor QP124. MOS transistorsQN125 and QN126 have gates connected to the power supply node, and arenormally kept on.

Bias voltage generating circuit 210 further includes N-channel MOStransistors QN127 and QN128 as well as a P-channel MOS transistor QP125.MOS transistors QN127 has a gate and a drain connected to each other,and receives a total current IA of the currents supplied from MOStransistors QP122 and QP123. MOS transistor QN128 form a current mirrorcircuit with MOS transistor QN127. MOS transistor QP125 has a gate and adrain connected to each other, supplies a current from the power supplynode to MOS transistor QN128 and converts the supplied current into avoltage to produce bias voltage PBIAS.

MOS transistors QN125 and QN126 are the same in size as transistors TR1and TR2 or transistors TR3 and TR4 in the series connection in unit cellUC of entry ERY . Therefore, MOS transistors QN125 and QN126 passone-bit pull-out current I_miss (=In) of the same magnitude as thecurrent discharged through the series connection of transistors in theon state of unit cell UC in the miss state.

This one-bit pull-out current I_miss is supplied via MOS transistorQP124. All MOS transistors QP121-QP125 are the same in size. Therefore,the current supplied from MOS transistor QP123 is pull-out currentI_miss (=In) of the one-bit unit cell in the miss state. MOS transistorQP121 supplies an off-leakage current for the replica unit cells UCh ofm bits to replica match line RMLb. Therefore, the current thus suppliedis equal to 2·m·I_off.

MOS transistor QP122 supplies a mirror current of the current suppliedby MOS transistor QP121. Therefore, the total current can be expressedby (2·I_off·m+I_miss (=m·Ioff+In)). MOS transistor QN127 discharges thiscurrent IA. The mirror current of current IA flows through MOStransistor QN128. MOS transistor QP125 produces bias voltage PBIAScorresponding to the current flowing through MOS transistor QN128.

MOS transistor QP120 is smaller in size (transconductance gm) than MOStransistor QP125. For example, MOS transistor QP120 has half times thetransconductance gm of MOS transistor QP125. MOS transistor QP120 passescurrent I_charge equal to or smaller than half the current IA. Thereby,match line ML can be supplied with a current that is smaller thanone-bit miss current Imiss or one-bit pull-out current I_miss (=In) butis larger than total off-leakage current IOFF (=2·m·I_off) of the entryin the match state.

As shown in FIG. 44, the variations in transistor parameters of unitcells UC in entry ERY can be reflected, using replica entry 200.Specifically, MOS transistors QN125 and QN126 are formed to be subjectedto the same parameter variations as transistors TR1-TR4 of unit cell UCin entry ERY (they are formed in a neighboring region through the samemanufacturing steps). The value of current IA can be adjusted accordingto the variations in pull-out current I_miss in the miss state. Forexample, when off-leakage current I_off of the unit cell in the entrystoring the data increases, current IA likewise increases. Accordingly,the current value of charge current I_charge supplied from chargecircuit 156 is increased, and the variations in off-leakage current formatch line ML can be compensated for. Therefore, even in the case whenmatch line ML is to be kept at the H level according to the searchresult in the last search cycle, match line ML can be accurately kept atthe H level.

When one-bit pull-out current I_miss (=In) lowers, total current IAlowers, and charge current I_charge can be reduced. Thereby, it ispossible to prevent the decrease of the lowering speed of the voltagelevel of the match line corresponding to the entry in the mismatchstate, so that the increases in search cycle can be prevented.

As described above, the thirteenth embodiment of the invention uses thereplica entry formed of the unit cells in the match state as well as thereplica unit cell corresponding to the one-bit miss state unit cell, toproduce the bias voltage for generating the charging current to thematch line. Therefore, the variations in transistor parameter can beaccurately compensated for, and the charging current of the match linecan be adjusted. The effects by the construction using the replica entryto generate the charging current to the match line are likewise achievedin the tenth and eleventh embodiments already described, and thevariations in transistor parameter can be compensated for to produce thecurrent at a desired level.

The same effects can be achieved as in the eleventh and twelfthembodiments.

Fourteenth Embodiment

FIG. 45 shows a main portion of a content addressable memory accordingto a fourteenth embodiment of the invention. The construction of thecontent addressable memory shown in FIG. 45 corresponds to a combinationof the tenth and thirteenth embodiments. In FIG. 45, match amplifier 150includes MOS transistors QP130, QP112 and QN130 connected in seriesbetween the power supply node and match line ML, D-type latch circuit154 for latching an internal search result signal MA_ML on a connectionnode between MOS transistors QP112 and QN130, and an EXNOR circuit 160for producing a charge instructing signal /CHRG. D-type latch circuit154 latches the signal received on its D-input according to a searchinstructing signal SRCH. EXNOR circuit 160 receives output signal ML_OUTof D-type latch circuit 154 and search instructing signal SRCH, andapplies charge instructing signal /CHRG to the gate of MOS transistorQP112.

MOS transistor QP130 has a gate receiving bias voltage BIAS_P from biasvoltage generating circuit 45. MOS transistor QN130 has a gate receivingbias voltage BIAS_N2 from buffer 130. These bias voltage generatingcircuits 45 and 130 have the same construction as bias voltagegenerating circuits 45 and 130 shown in FIG. 31.

Memory cell array has a plurality of entries ERY , and match lines MLare arranged corresponding to entries ERY , respectively. Entry ERYincludes unit cells UC of m bits. Unit cell UC has the same constructionas the unit cells in the first to twelfth embodiments, and includes aCAM cell CC for storing the data and search MOS transistors TR1-TR4 forcomparing the search data and the stored data.

Bias voltage generating circuit 45 shown in FIG. 45 produces biasvoltage BIAS_P to the gate of P-channel MOS transistor QP130 included inmatch amplifier 150. MOS transistor QP130 supplies a current smaller invalue than the current (I_miss or Imiss) flowing through the one-bitmiss state unit cell, and is larger than the total (IOFF) of off-leakagecurrents I_off of the unit cells of m bits in the match state in thecorresponding entry. P- and N-channel MOS transistors QP112 and QN130are connected in series between MOS transistor QP130 and match line ML.MOS transistor QN130 receives bias voltage BIAS_N2 from buffer 130. MOStransistors QP130 and QN130 correspond to MOS transistors QP91 and QN71shown in FIG. 31, respectively.

In the construction shown in FIG. 45, MOS transistor QN130 operates inthe source follower mode in according to bias voltage BIAS_N2.Therefore, corresponding match line ML attains the voltage level equalto or lower than the voltage VML (≦DD/2), and the voltage rising of thematch line is suppressed. Accordingly, the potential of internal nodeMA_ML rises fast, and the determination result signal MA_ML can bedriven fast to the definite state (power supply voltage level).

In the construction shown in FIG. 45, the signal /CHRG to MOS transistorQP112 is produced according to search result indicating signal ML_OUT inthe last cycle. Therefore, it is possible to control supply/cut-off ofthe charging current to corresponding match line ML in the searchoperation according to the result of search/determination in the lastsearch cycle.

FIG. 46 shows an operation of a content addressable memory shown in FIG.45. Referring to FIG. 46, the operation of the content addressablememory shown in FIG. 45 will now be described.

In the search operation shown in FIG. 46, two cycles of clock signal CLKdefine one search cycle. However, the search cycle may be equal to onecycle of clock signal CLK, similarly to the twelfth embodiment.

It is assumed that the result of the determination in the last searchcycle indicates the mismatch state in cycles CY1 and CY2 of clock signalCLK. According to this assumption, match line ML is kept at the groundvoltage level, internal search result signal MA_ML is at the groundvoltage level and output signal ML_OUT of D-type latch circuit 154 is atthe L level.

In a clock cycle CY3, the search operation is performed according to newsearch data. In this cycle, when search instructing signal SRCH attainsthe H level, EXNOR circuit 160 produces the signal /CHRG at the L level.Accordingly, MOS transistor QP112 is turned on to charge match line ML.In the operation of charging the match line ML, MOS transistor QN130suppresses the voltage level of match line ML to or below intermediatevoltage VML, as described in the tenth embodiment. MOS transistor QN130performs a decoupling operation according to bias voltage BIAS_N2applied from buffer 130. Thus, the voltage level of internal searchresult signal MA_ML rises, and finally attains the level of power supplyvoltage VDD.

In a clock cycle CY4, when search instructing signal SRCH attains the Llevel, search result indicating signal ML_OUT applied from D-type latchcircuit 154 rises to the H level. At this time, output signal /CHRG ofEXNOR circuit 160 is held at the L level, and the charging operation isperformed on the match line. However, the source follower operation ofMOS transistor QN130 holds match line ML at the level of voltage VML orlower. Thus, match line ML is kept at the predetermined voltage level,using the off-leakage currents of the unit cells of all bits on matchline ML.

In a clock cycle CY5, the search data changes, and search instructingsignal SRCH attains the L level again. Since the search result in thelast cycle indicates the match state, signal ML_OUT is at the H level,and EXNOR circuit 160 produces the output signal /CHRG at the H level,so that MOS transistor QP112 is turned off to stop the charging of matchline ML. In this cycle, when the search result of the search dataindicates the mismatch state, match line ML is discharged to lower itsvoltage level. Accordingly, the voltage level of internal search resultsignal MA_ML lowers.

In a clock cycle CY6, when search instructing signal SRCH falls to the Llevel, D-type latch circuit 154 produces output signal ML_OUT at the Llevel. Output signal /CHRG of EXNOR circuit 160 keeps the H level, andthe charging of match line ML is stopped. Therefore, match line ML iskept at the ground voltage level.

Then, the search is performed according to the next search data in aclock cycle CY7. In the case where the result of search for the searchdata indicates the mismatch state, the charging current is dischargedvia the unit cell in the miss state of the entry coupled to match lineML even when EXNOR circuit 160 produces output signal /CHRG at the Llevel. Therefore, match line ML keeps the ground voltage level.Accordingly, search result indicating signal ML_OUT likewise attains theL level according to the voltage level of internal signal line MA_ML.

In a clock cycle CY8, when search instructing signal SRCH attains the Llevel, MOS transistor QP112 is turned on. Responsively, match line ML ischarged to raise its voltage level. Also, the voltage level of internalsearch result signal MA_ML rises.

In a clock cycle CY10, D-type latch circuit 150 produces output signalML_OUT at the H level. In this case, search instructing signal SRCH isat the L level, and output signal /CHRG of EXNOR circuit 160 keeps the Llevel and match line ML is charged. Accordingly, match line ML is keptat the level of intermediate voltage (precharge voltage) VML.

In a clock cycle CY11, the search operation is performed again. Whensearch result is the match state, EXNOR circuit 160 produces outputsignal /CHRG at the H level to stop the charging of match line ML. Whensearch instructing signal SRCH attains the L level, the signal /CHRGattains the L level. Responsively, match line ML is charged through MOStransistor QN130, and keeps its voltage level.

In a clock cycle CY13, the search operation is performed again. When thesearch result indicates the mismatch state, the signal /CHRG is at the Hlevel, and the discharging of match line ML is not performed.Accordingly, the voltage levels of match line ML and internal signalline MA_ML lower to the ground voltage level, and search resultindicating signal ML_OUT lowers to the L level in a next clock cycleCY14. At this time, the signal /CHRG attains the L level again, andmatch line ML is charged.

Therefore, when the last search result indicates the match state, thecurrent supplying to match line ML is stopped in the search operation.Only when the last search result indicates the mismatch state, thecharge current is supplied to the match line in the search operation.Thereby, the number of times of charging match line ML can be reduced,and the current consumption can be reduced. When the search result inthe last cycle indicates the mismatch state, match line ML is at theground voltage level and it may possibly need to raise its voltagelevel, the current is supplied to the match line in the searchoperation, and thus the search operation can be performed fast.

For the match line, it is not necessary to perform the precharging ineach search cycle, and the current consumption can be reduced. Similarlyto the tenth embodiment, the quantity of charging current of match lineML is adjusted using bias voltages BIAS_P and BIAS_N2, and the chargingcurrent is supplied to the match line via MOS transistor QN130.Therefore, the voltage level of match line ML can be restricted to thelevel of voltage VML or lower. Accordingly, the voltage amplitude of thematch line can be reduced, and the current consumption in the searchoperation can be significantly reduced, similarly to the tenthembodiment. Since the sensing of the match and mismatch states areperformed according to the charging/discharging of internal signal lineMA_ML of a small interconnection load, the fast search operation can beachieved.

Meanwhile, in the fourteenth embodiment, the current converter circuitshown in FIG. 37 may be provided for level-converting bias voltageBIAS_P.

Fifteenth Embodiment

FIG. 47 schematically shows a construction of a content addressablememory according to the fifteenth embodiment of the invention. In FIG.47, a CAM cell array including CAM cells (unit cells) is divided intotwo global search blocks GSB1 and GSB2. Although not clearly shown, theCAM cell array includes a plurality of entries as already described.Global search block GSB1 includes a plurality of local search blocksSB11-SB18, and global search block GSB2 includes a plurality of localsearch blocks SB21-SB28. Each of local search blocks SB11-SB18 andSB21-SB28 is provided with a match line group MLs formed of a pluralityof match lines ML, and is also provided with a search line group (searchdata bus) SLPs formed of a plurality of search line pairs. Each matchline group MLs includes match lines ML of 1 K in number. Search linepair group SLPs includes search line pairs SLP of 144 bits.

For local search blocks SB11-SB18, there are provided match amplifiergroups MA11-MA18 as well as priority encoders PE11-PE18, respectively.Each of match amplifier groups MA11-MA18 includes match amplifiersarranged corresponding to the match lines in the corresponding localsearch blocks, respectively. The match amplifier may be any one of thematch amplifiers in the first to thirteenth embodiments, or may also besubstantially the same as a conventional match amplifier. By using thematch amplifier in any embodiment already described, the searchoperation can be performed fast with low current consumption. Even whenthe match amplifier the same in construction as the conventional matchamplifier is employed, it can achieve the effects of the construction ofthe content addressable memory according to the fifteenth embodiment perse.

Each of priority encoders PE11-PE18 selects the match line of thehighest priority from corresponding match amplifier groups MA11-MA18according to a predetermined priority rule, and produces the informationon the selected match line.

In FIG. 47, local search block SB11 has the highest priority, and thepriority decreases as approaching local search block SB18. Global searchblock GSB1 has a higher priority than global search block GSB2. Inglobal search block GSB2, the priority successively lowers asapproaching local search block SB28 from local search block SB21.

For local search blocks SB11-SB18, search data input circuits FF11-FF18each formed of a flip-flop are arranged, respectively. These search datainput circuits FF11-FF18 commonly receive the search data from an FFcircuit PP 1 receiving externally applied search data SD. Search datainput circuits FF11-FF18 drive search line groups SLs of correspondingsearch blocks SB11-SB18 according to the received search data,respectively.

Search data input circuit (FF circuit) FF1 takes in and outputs thereceived data according to an externally applied clock signal CLKex.Therefore, the search data supplied to search data input circuits (FFcircuits) FF11-FF18 are updated in each cycle of external clock signalCLKex.

Global search block GSB1 is further provided with cascaded digital delaycircuits DL10-DL18 for successively delaying external clock signalCLKex. Delay clock signals supplied from digital delay circuitsDL10-DL17 are applied to search data input circuits FL11-FL18 as searchdata take-in clock signals, respectively. According to the delay clocksignals applied from digital delay circuits DL11-DL18, respective localsearch blocks SB11-SB18 execute the search, determination of searchresult and production of search result indication. For example, theoutput signals of digital delay circuits DL11-DL18 are used as searchinstructing signal SRCH as described in the twelfth embodiment.Accordingly, search blocks SB11-SB18 perform the search, determinationof the search result and output of search result determination with thestart timing staggered successively.

In global search block GSB2, search data input circuits FF21-FF28 arearranged corresponding to local search blocks SB21-SB28, respectively.Also, match amplifier groups MA21-MA28 and priority encoders PE21-PE28are also arranged corresponding to local search blocks SB21-SB28,respectively. Digital delay circuits DL20-DL28 concatenated together arearranged for controlling the search operations of the respective searchblocks.

For adjusting the delay times of digital delay circuits DL10-DL18 andDL20-DL28, a delay control circuit 220 is arranged.

Delay control circuit 220 includes a digital phase difference detectingcircuit 222 for detecting a phase difference between external clocksignal CLKex and an internal clock signal CLKin to produce a delaycontrol signal corresponding to a phase difference, cascaded digitaldelay circuits DL1-DL8, and an inverter buffer IVB for inverting theoutput signal of digital delay circuit DL8 in the final stage to applythe inverted signal to digital delay circuit DL1 in the initial stage.Inverter buffer IVB and digital delay circuits DL1-DL8 form a ringoscillator. Digital delay circuit DL8 produces internal clock signalCLKin.

Digital phase difference detecting circuit 222 adjusts the delay timesof digital delay circuits DL1-DL8 to equalize the phases of externalclock signal CLKex and internal clock signal CLKin, to adjust the delaytimes of digital delay circuits DL10-DL18 and DL20-DL28 providedcorresponding to the respective local search blocks.

For global search block GSB2, there are arranged a search data inputcircuit (FF circuit) FF2 receiving the search data supplied from searchdata input circuit (FF circuit) FF1 as well as a gate circuit 225receiving the output signal of priority encoder PE18 and external clocksignal CLKex.

Priority encoder PE18 has the lowest priority in the global searchblock. Therefore, when priority encoder PE18 generates a signal at the Llevel, this indicates the state in which stored data matching the searchdata is not found in global search block GSB1. Therefore, global searchblock GSB2 executes the search operation when the match detection is notaccomplished in global search block GSB1.

The output signal of gate circuit 225 is applied as a clock signal todigital delay circuits DL20-DL28. When gate circuit 225 is enabled (whenpriority encoder PE18 produces the output signal at the L level), localsearch blocks SB21-SB28 in global search block GSB2 successively performthe search operation according to the staggered operation start timingin accordance with the clock signal applied via gate circuit 225.

Search data input circuit (FF circuit) FF2 likewise takes in the searchdata supplied from FF circuit FF1 according to the output signal of gatecircuit 225, and applies the data thus taken in, as the search data, tosearch data input circuits FF21-FF28 of local search blocks SB21-SB28.

FIG. 48 is a timing chart representing the operation of delay controlcircuit 220 shown in FIG. 47. Referring to FIG. 48, the operation ofdelay control circuit 220 shown in FIG. 47 will now be described.

As described above, digital phase difference detecting circuit 222adjusts the delay times of digital delay circuits DL1-DL8 to equalizethe phases of external clock signal CLKex and internal clock signalCLKin.

When clock signals CLKex and CLKin are the same in phase, the outputsignal of inverter buffer IVB changes in synchronization with externalclock signal CLKex according to internal clock signal CLKin. The delaytime of inverter buffer WB, in this example, assumes a negligible valueas compared with the delay times of digital delay circuits DL1-DL8. Whenthe output signal of inverter buffer IVB changes, each of digital delaycircuits DL1-DL8 changes the logical state of its output signal with adelay time d. Therefore, when internal and external clock signals CLKinand CLKex matches in phase with each other, a cycle time Tc of internalclock signal CLKin is equal to d·2·8 (internal and external clocksignals CLKin and CLKex have the same cycle period).

Digital delay circuits DL1-DL8 and digital delay circuits DL10-DL18 andDL20-DL28 have the same constructions. Therefore, the same delay time asthe delay time of digital delay circuits DL1-DL8 can be set in digitaldelay circuits DL10-DL18 and DL20-DL28.

Each of digital delay circuits DL1-DL8, DL10-DL18 and DL20-DL28 isformed of a buffer circuit (inverters in two stages) having a variablecurrent source that can change an operation current. The operationcurrent value of each digital delay circuit is adjusted according to thephase difference information detected by digital phase differencedetecting circuit 222, and thereby delay time, d, is adjusted.

As described above, the clock signal for defining the operation cyclesof local search blocks SB11-SB18 is supplied using digital delaycircuits DL10-DL18. Thus, in global search block GSB1, the searchoperation and search result determining operation can be successivelyperformed by local search blocks SB11-SB18 with the delay time d ofdigital delay circuits DL10-DL18, respectively, and the peak current canbe small. The same holds for global search block GSB2.

The search lines are divided according to the local search blocks sothat the interconnection load capacitance of the search line can besmall, and the charging/discharging currents of the search line can besmall. Further, the search line can be driven fast according to thesearch data.

FIG. 49 is a timing chart representing the operation of the contentaddressable memory shown in FIG. 47. Referring to FIG. 49, the operationof the content addressable memory shown in FIG. 47 will now bedescribed.

As shown in FIG. 49, when external clock signal CLKex attains the Hlevel, the output signal of search data input circuit (FF circuit) FF1is updated and decided. Then, search data input circuit FF11 for localsearch block SB11 takes in the received data according to the outputsignal of digital delay circuit DL10, and drives internal search dataline group SLPs according to the search data. Accordingly, matchamplifier group MA11 and priority encoder PE11 are made active accordingto the output signal of digital delay circuit DL11 after elapsing ofdelay time d of digital delay circuit DL11 and the search data iscompared with the stored data, and the search result is determined.

In parallel with the match line driving in local search block SB11, inlocal search block SB12, supplied data is taken in by search data inputcircuit FF12 according to the output signal of digital delay circuitDL11, corresponding search data line group SLPs is driven according tothe search data. Subsequently, the search operation is internallyperformed in search block SB11 according to the output signal of digitaldelay circuit DL12 after elapsing of time d.

Thereafter, local search blocks SB11, . . . SB17 (not shown)successively execute the search operation. Each of search data inputcircuits FF11-FF17 takes in the search data supplied from search datainput circuit FF1 in synchronization with the rising of the outputsignal of the corresponding digital delay circuit (the rising ofexternal clock signal CLKex), and enters the latch state. In localsearch block SB18 in the final stage, search data input circuit FF18enters the latch state according to the output signal of digital delaycircuit DL17 (not shown) in the preceding stage, and drives search linegroup SLBs according to the search data. Then, match amplifier groupMA18 is activated when time d elapses.

The priority successively lowers in the direction from local searchblock SB1 toward local search block SB18. In any one of the local searchblocks in global search block GSB1, when the match state is detected,the output signal of priority encoder PE18 in the final stage attainsthe H level, and the output signal of gate circuit 225 is fixed to the Llevel. This inhibits the transmission of the clock signal to globalsearch block GSB2, and stops the search operation therein. At this time,search data input circuit (FF circuit) FF2 does not latch the receivedsearch data.

The search lines are divided into a plurality of global blocks toimplement a block division construction. When the match is detected inthe global search line block of a higher priority, the search operationof the global search line block of a lowrer priority is stopped.Accordingly, the current consumption can be reduced.

FIG. 50 shows an example of constructions of priority encoders PE11-PE18and PE21-PE28. In FIG. 50, priority encoder indicated by PE is arepresentative example of the constructions of priority encodersPE11-PEI 8 and PE21-PE28. FIG. 50 shows, by way of example, anarrangement of the priority encoder having the highest priority.

In FIG. 50, determination result outputs MLOUTa-MLOUTn correspond to thesearch result indicating signals produced for the respective match linesfrom the match amplifiers in the corresponding match amplifier group.

Priority encoder PE includes gate circuits GTa-GTn arrangedcorresponding to determination result outputs MLOUTa-MLOUTn of the matchlines, respectively, and OR gates OGa-OGn receiving the output signalsof gate circuits GTa-GTn, respectively. Gate circuits GTa-GTn receive,on the complementary inputs, the output signals of corresponding ORgates OGa-OG(n−1) in the preceding stage, and receive, on thenon-inverted inputs, the output signals of the corresponding matchamplifiers. OR gate OGn in the final stage applies the output signal topriority encoder PE of the next higher priority. The complementary inputof gate circuit GTa is coupled to the ground node.

Output MLOUTa has the highest priority, and output MLOUTn has the lowestpriority. Each of gate circuits GTa-GTn produces the signal at the Hlevel when search result indication MLOUT received from thecorresponding match line is at the H level indicating the match stateand the output signal of the corresponding OR gate is at the L level.Each of gate circuits GTa-GTn fixes its output signal at the L levelwhen the signal received at its complementary input attains the H level.

The complementary input of gate circuit GTa is fixed at the groundvoltage level (because the shown priority encoder has the highestpriority). However, when a local search block of a higher priority isarranged with respect to priority encoder PE, the complementary input ofgate circuit GTa of priority encoder PE is supplied with an outputsignal of OR gate OGn in the final stage of the priority encoder in thepreceding stage, instead of the ground voltage.

Match line data MLDTa-MLDTn supplied from gate circuits GTa-GTn may befurther encoded to produce information designating the address of thematch line in the match state. Match line data MLDTa MLDTn may be usedas word line drive signals to drive a word line of a table memory to theselected state so that the data may be read from the corresponding wordline (the outputs of gate circuits GTa-GTn are coupled to the word linesof the table memory).

In priority encoder PE shown in FIG. 50, it is assumed that outputsignals MLOUTb and MLOUTc are both at the H level, and signal MLOUTa isat the L level. In this case, gate circuit GTa produces the outputsignal at the L level, and OR gate OGa produces the output signal at theL level. Therefore, gate circuit GTb produces match line data MLDTb atthe H level. When match line data MLDTb attains the H level, OR gate OGbproduces the output signal at the H level. In this state, gate circuitGTc holds its output signal MLDTc at the L level even when correspondingmatch line data MLOUTc is at the H level. All the output signals of theOR gates of lower priorities are at the H level, and the outputs of thegate circuits of the lower priorities attain the L level. Therefore,when the match is detected from the match line of a higher priority, thematch line information on the match line of the detected highestpriority is driven to the active state, and the other match line data isset to indicate the search-miss state.

The priority encoder shown by way of example in FIG. 50 is used for eachof priority encoders PE11-PE18 shown in FIG. 47. Accordingly, when thepriority encoder of the higher priority among priority encodersPE11-PE18 shown in FIG. 47 detects the match, the match line datasupplied from the priority encoders of priorities lower than thedetected priority are all kept at the L level indicating the mismatchstate (the output signal of OR gate is set to the H level).

The OR gate in the final stage of priority encoder PE18 provides theoutput signal at the H level when the match state is sensed in globalsearch block GSB1. Thereby, the output signal of gate circuit 225 shownin FIG. 47 is fixed at the L level to inhibit the transmission of theclock signal to global search block GSB2. Since the search operation inglobal search block GSB2 is inhibited, the number of the operating localsearch blocks can be reduced, and the current consumption can bereduced.

In the construction shown in FIG. 47, each of global search blocks GSB1and GSB2 includes eight local search blocks. However, the local searchblocks included in one global search block are not restricted to suchnumber of eight, and another number of local search blocks may be used.Also, more global search blocks than two global search blocks GSB1 andGSB2 may be employed. The number of the digital delay circuits includedin delay control circuit 220 is set according to the number of the localsearch blocks included in the global search block.

The circuit for generating the bias voltage or the reference voltage maybe arranged commonly to the global search blocks, and may also bearranged for each global search block separately and individually. Forthe control signal controlling the activation and search operation ofthe local search block, when search instruction signal SRCH may beemployed as is done in the eleventh to fourteenth embodiments, thesignal supplied from the digital delay circuit can be used as suchsearch instructing signal. When a match amplifier activating signal, aprecharge instructing signal and/or the like are used alternatively, itis sufficient to produce the operation control signal for each localsearch block with the operation timing adjusted based on the outputsignals of the corresponding digital delay circuits.

Sixteenth Embodiment

FIG. 51 schematically shows a main portion of a content addressablememory according to a sixteenth embodiment of the invention. FIG. 51representatively shows a match amplifier 200 arranged corresponding tomatch line ML. Match line ML is coupled to unit cells UC arranged inparallel. FIG. 61 shows, by way of example, search data of 73 bits onsearch lines SL[0] and /SL[0]-SL[72] and /SL[72].

In unit cell UC of the construction shown in FIG. 51, the CAM cellstoring the data is formed of an SRAM cell, which is connected to gatesof MOS transistors TR1 and TR3 nearer to match line ML. MOS transistorsTR2 and TR4 are coupled to search lines SL[0] and /SL[0], respectively.Alternatively, unit cell UC may be configured such that the SRAM cell iscoupled to MOS transistors TR2 and TR4, and search lines SL[0] and/SL[0] is coupled to MOS transistors TR1 and TR3, respectively. Twomemory cells may be used in place of one SRAM cell SMC to implement: aconstruction of the unit cell storing ternary data. The construction ofunit cell UC is not restricted to that shown in FIG. 51, and theconstruction of the unit cell in the first embodiment shown in FIGS. 2and 3 may be employed.

Match amplifier 200 includes a precharge circuit 210 and an isolationgate circuit 30. Precharge circuit 210 precharges match line ML andreference voltage node NDa to the level of precharge voltage VML notexceeding the intermediate voltage according to a precharge instructingsignal MLPRE. Isolation gate circuit 30 isolates match line ML andreference voltage node NDa from internal nodes (first and second nodes)NDb and NDc according to isolation instructing signal MLI, respectively.

Precharge circuit 210 includes N-channel MOS transistors 211 and 212that transmit precharge voltage VML to match line ML and referencevoltage node NDa according to precharge instructing signal MLPRE,respectively. Isolation gate circuit 30 includes transfer gates TGa andTGb arranged for match line ML and reference voltage node NDa,respectively.

Match amplifier 200 further includes amplifier circuit 12 and latch 16.Amplifier circuit 12 is made active in response to the activation ofmatch amplifier activating signal MAE, to differentially amplify thesignals ML_MA and MLREF on internal nodes NDb and NDc. Latch 16 latchesthe output signal of amplifier circuit 12 according to latch instructingsignal LAT.

Amplifier circuit 12 is substantially the same in construction asdifferential amplifier circuit 12 shown in FIG. 10. Amplifier circuit 12differentially amplifies and latches voltages ML_MA and MLREF oninternal nodes NDb and NDc. Latch 16 is the same in construction as thelatch shown in FIG. 10. When latch instructing signal LAT is at the Hlevel, latch 16 attains the through state, and produces search resultindicating signal ML_OUT according to the output signal of amplifiercircuit 12.

Match amplifier 200 further includes capacitance elements C1_A and C1_Bprovided corresponding to first and second internal nodes NDb and NDc,respectively. Capacitance element C1_A boosts the voltage level ofinternal node NDb through the charge pump operation (capacitivecoupling) according to a boost instructing signal MLUP. Capacitanceelement C1_B is connected between internal node NDc and the power supplynode. Capacitance element C1_B is the same in capacitance value ascapacitance element C1_A, and is employed for balancing a capacitanceload between internal nodes NDb and NDc.

FIG. 52 is a timing chart representing an operation of the contentaddressable memory shown in FIG. 51. Referring to FIG. 52, the searchoperation of the content addressable memory shown in FIG. 51 will now bedescribed.

Before starting the search operation, match line ML is at the level ofground voltage GND. Isolation instructing signal MLI is at the H level,and isolation gate circuit 30 is conductive. Latch instructing signalLAT is at the L level, and latch 16 is in the latch state and producesthe signal ML_OUT at the H level, for example.

At time T1, the search cycle starts. According to the start of thesearch operation, precharge instructing signal MLPRE is kept at the Hlevel for half the cycle period of clock signal CLK. Accordingly, matchline ML and reference voltage node NDa are precharged to the level ofintermediate voltage VML. Precharge voltage VML is at the level betweenintermediate voltage VDD/2 and the ground voltage. Through the prechargeoperation by precharge circuit 210, the voltages ML_MA and MLREF ofinternal nodes NDb and NDc each are also precharged to the intermediatevoltage level.

When clock signal CLK falls to the L level at time T2, prechargeinstructing signal MLPRE attains the L level. Responsively, prechargecircuit 210 is made inactive to complete the operation of prechargingthe match line ML and reference voltage node NDa. After the completionof the precharge operation, the search data is transmitted onto searchlines SL and /SL (generically representing SL[0] and /SL[0]-SL[72] and/SL[72]), and the voltage levels thereon change according to the searchdata. When the search data does not match the stored data of unit cellsUC coupled to match line ML (in the case of the mismatch state), matchline ML is discharged via at least one unit cell UC, and has the voltagelevel thereof lowered. According to this voltage level lowering of matchline ML, the level of voltage ML_MA on internal node NDb lowers, VoltageMLREF on internal node NDc is at the level of intermediate voltage VML.

When the voltage level of match line ML sufficiently lowers, isolationinstructing signal MLI attains the L level to turn isolation gatecircuit 30 non-conductive at time T3.

In response to the non-conduction of isolation gate circuit 30, boostinstructing signal MLUP is driven to the H level. Internal node NDb isin the electrically floating state (match amplifier activationinstructing signal MAE is inactive). Therefore, through the capacitivecoupling by capacitance element C1_A, the level of voltage ML_MA on nodeNDb rises. The increase of voltage ML_MA on internal node NDb may beequal to or smaller than half an absolute value (|VML−GND|) of adifference between intermediate voltage VML and ground voltage GND. Inthe case of the mismatch state, this boosting operation reliablymaintains the voltage difference between voltage ML_MA and prechargevoltage VML on respective nodes NDb and NDc.

Then, match amplifier activating signal MAE is activated. Responsively,amplifier circuit 12 is activated to amplify differentially and latchthe voltages ML_MA and MLREF. In the mismatch state, the voltage MLREFon node NDc is driven to the level of power supply voltage VDD, andvoltage ML_MA on internal node NDb is driven to the level of groundvoltage. When the amplifying operation of amplifier circuit 12 is madeactive, latch instructing signal LAT attains the H level. Responsively,latch 16 enters the through state, and search result indicating signalML_OUT produced from latch 16 attains the L level indicating themismatch state.

At time T4, the search operation is completed, and search lines SL and/SL are precharged to the ground voltage level. Latch instructing signalLAT is driven to the L level to keep latch 16 in the latch state. Whenlatch 16 enters the latch state, match amplifier activating signal MAEbecomes inactive, and amplifier circuit 12 is made inactive. Then, boostinstructing signal MLUP is driven to the L level, and subsequentlyisolation instructing signal MLI is driven to the H level to renderisolation gate circuit 30 conductive. When boost instructing signal MLUPis driven to the L level, through the capacitive coupling of capacitanceelement C1_A, the voltage level of internal node NDb drops. However,when isolation gate circuit 30 is conductive, due to the chargessupplied from match line ML at the ground voltage level, the voltagelevel of internal node NDb is held at the ground voltage level.

Voltage MLREF on internal node NDc keeps the level of power supplyvoltage VDD amplified by amplifier circuit 12.

When a next search cycle starts at time T5. Precharge instructing signalMLPRE attains the H level, to precharge match line ML and referencevoltage node NDa to the level of precharge voltage VML.

At time T6, the search data is transmitted to search lines SL and /SL tochange the voltage levels of search lines SL and /SL.

When the search data matches the data held in all unit cells UC in thecorresponding entry, the discharging path is not present for match lineML, and match line ML is kept at the level of precharge voltage VML.

When clock signal CLK rises to the H level at time T7, isolationinstructing signal MLI is driven to the L level to turn isolation gatecircuit 30 non-conductive. Subsequently, boost instructing signal MLUPis driven to the H level. Responsively, through the capacitive couplingof capacitance element C1_A, the level of voltage ML_MA on internal nodeNDb is raised to exceed precharge voltage VML. Accordingly, a voltagedifference is developed between internal nodes NDb and NDc. According tothe activation of match amplifier activating signal MAE, amplifiercircuit 12 can differentially amplify and latch these voltages ML_MA andMLREF. When latch instructing signal LAT is then driven to the H level,latch 16 enters the through state, to drive output signal ML_OUT to theH level indicative of the match state.

When clock signal CLK falls at time T8, the match determination periodexpires, and latch instructing signal LAT attains the L level. Then,match amplifier activating signal MAE becomes inactive. Accordingly,boost instructing signal MLUP is driven to the L level, and subsequentlyisolation instructing signal MLI is driven to the H level. Responsively,isolation gate circuit 30 turns conductive to couple internal node NDbto match line ML.

When isolation gate circuit 30 turns conductive to couple match line MLto internal node NDb, the voltage on match line ML attains the samevoltage level as signal voltage ML_MA on internal node NDb. FIG. 52 doesnot show this voltage change. When the next search cycle starts,internal node NDb and match line ML are driven to the level of prechargevoltage VML according to precharge instructing signal MLPRE. Since aparasitic capacitance of match line ML is much larger than that ofinternal node NDb, voltage ML_MA lowers to the voltage level close toprecharge voltage VML when isolation gate circuit 30 turns conductive.In this case, isolation gate circuit 30 may have a decouple function toallow transmission of up to intermediate voltage VML. (A differencebetween the H level of isolation instructing signal MLI and thethreshold voltage of isolation gate TGa is set to a level ofintermediate voltage VML.)

FIG. 53 schematically shows a whole construction of the contentaddressable memory according to the sixteenth embodiment of theinvention. The content addressable memory shown in FIG. 53 issubstantially the same in construction as the content addressable memoryshown in FIG. 1 except for the reference numerals or characters.Specifically, memory cell array 1 includes unit cells UC arranged inrows and columns, and is divided into a plurality of entries ERY . Matchlines ML are arranged corresponding to the respective entries ERY , andmatch amplifiers 200 are arranged corresponding to the respective matchlines ML.

Match amplifier 200 arranged corresponding to each match line MLconfigures match determining circuit 2. An intermediate voltagegenerating circuit 222 commonly supplies, as a precharge voltage, theintermediate voltage VML to match amplifiers 200.

Control circuit 220 performs the operation control so as to achieve theoperation instructed by command CMD that is supplied in synchronizationwith clock signal CLK.

FIG. 54 schematically shows an example of a construction of controlcircuit 220 shown in FIG. 53. Control circuit 220 in FIG. 54 is the samein construction as the control circuit shown in FIG. 18. Specifically,control circuit 220 in FIG. 54 includes command decoder 20, a prechargeactivating circuit 230, a search line drive activating circuit 232, adelay circuit 234 and a match amplifier activating circuit 236,similarly to the construction shown in FIG. 18.

Command decoder 20 decodes externally supplied command CMD insynchronization with clock signal CLK. Precharge activating circuit 230produces precharge instructing signal MLPRE according to searchoperation instruction EN received from command decoder 20 and clocksignal CLK. When search operation instruction EN is active, search linedrive activating circuit 232 holds search line activating signal SLENactivating the search line in the active state for a period of the Llevel of clock signal CLK. Delay circuit 234 delays search operationinstruction EN by one clock cycle of clock signal CLK.

Match amplifier activating circuit 236 sequentially activates isolationinstructing signal MLI, boost instructing signal MLUP, match amplifieractivating signal MAE and latch instructing signal LAT in this order inresponse to rising of clock signal CLK to the H level when the outputsignal of delay circuit 234 is active. When clock signal CLK falls then,match amplifier activating circuit 236 first deactivates latchinstructing signal LAT, then deactivates match amplifier activatingsignal MAE and thereafter sequentially deactivates boost instructingsignal MLUP and isolation instructing signal MLI.

Match amplifier activating circuit 236 is formed of, e.g., a series ofset/reset flip-flops, which are sequentially driven to the set state,and then are sequentially deactivated. With such construction, thecontrol signals shown in FIG. 52 for match amplifiers 200 can beactivated and deactivated in a predetermined sequence.

According to the sixteenth embodiment of the invention, as describedabove, the amplifying operation is performed while the match line isisolated from the amplifier circuit of the match amplifier to confinethe charges. The capacitance element for boosting the internal node(first node) connected to the amplifier circuit is not required to boostan entire of a match line, so that the capacitance element can have areduced size, and the occupation area of the match amplifier can besmall. Also, the boost instructing signal generating portion (matchamplifier activating circuit 236) for driving the capacitance element isjust required to drive a small capacitance, and the load to be driventhereby is small so that the current consumption can be small. Since theamplification is performed according to the charge confining scheme, theload of the amplifier circuit is small, and the load capacitance of thenode having the voltage full-swinging is small, and accordingly thecurrent consumption in the amplifying operation can be small. Further,the amplifying operation can be performed fast.

According to the timing chart shown in FIG. 52, match line ML isprecharged to the intermediate voltage level only at the start of thesearch. However, this precharge operation may be configured to beperformed during standby before the search operation and to be completedat the start of the search operation.

Seventeenth Embodiment

FIG. 55 schematically shows a main portion of a content addressablememory according to a seventeenth embodiment of the invention. Thecontent addressable memory shown in FIG. 55 differs from the contentaddressable memory shown in FIG. 51 in the following construction. Inmatch amplifier 200, a capacitance element C2_A is connected at oneelectrode to internal node NDb coupled to match line ML, and to theground at the other electrode. Capacitance element C2_A functions as acapacitance for accumulating charges transmitted from match line ML. Acapacitance element C2_B is provided for internal node NDc producing thereference voltage. Capacitance element C2_B receives a reference voltagedown instructing signal REFDOWN on its other electrode, and lowersreference voltage MLREF on internal node NDc through its capacitivecoupling (charge pump operation).

Other constructions of the content addressable memory shown in FIG. 55are the same as those shown in FIG. 51. Corresponding portions areallotted the same reference numerals, and description thereof is notrepeated.

FIG. 56 is a timing chart representing a search operation of theconstruction shown in FIG. 55. Referring to FIG. 56, the operation ofthe circuits shown in FIG. 55 will now be described.

The operations from time T1 to time T3 are the same as those of thesearch operation in the sixteenth embodiment represented in FIG. 52.Before the search data is transferred to search lines SL and /SL, matchline ML and reference voltage node NDa are charged to the voltage levelof intermediate voltage VDD/2 or lower (according to on match lineprecharge instructing signal MLPRE at time T1). At this time, isolationgate circuit 30 is conductive, and internal node NDc is likewiseprecharged to the level of precharge voltage VML of intermediate voltageVDD/2 or lower.

At time T2, the search data is transferred. Upon mismatch, match line MLis discharged to have a lowered voltage level. Isolation gate circuit 30in conductive, and accordingly voltage ML_MA on internal node NDblowers. Reference voltage MLREF on internal node NDc is kept at thelevel of precharge voltage VML.

AT time T3, isolation instructing signal MLI is driven to the L level toturn isolation gate circuit 30 non-conductive. Then, reference voltagedown instructing signal REFDOWN is driven from the H level to the Llevel. Responsively, through capacitive coupling of capacitance elementC2_B, the voltage MLREF on internal node NDc lowers. This capacitancevalue of capacitance element C2_B is determined such that the voltage oninternal node NDc may attain the level equal to or lower than half adifference of |VML−GND| between precharge voltage VML and ground voltageGND.

The voltage MLREF on internal node NDc does not lower below the level ofvoltage ML_MA on internal node NDb. Then, match amplifier activatingsignal MAE is made active to activate amplifier circuit 12. Throughdifferential amplification of amplifier circuit 12, voltage ML_MA oninternal node NDb is driven to the ground voltage level. Also, referencevoltage MLREF on internal node NDc is driven to and latched at the levelof power supply voltage VDD. Then, latch instructing signal LAT is madeactive to set latch 16 to the through state so that output signal ML_OUTis driven to the L level indicating the mismatch state.

At time T4, this determining operation is completed, and latchinstructing signal LAT is driven to the L level. Responsively, latch 16enters the latch state, and output signal ML_OUT is kept at the L level.Then, match amplifier activating signal MAE is driven to the L level todeactivate amplifier circuit 12.

Thereafter, reference voltage down instructing signal REFDOWN is drivento the L level. At this time, internal node NDc is in the electricallyfloating state, and the voltage MLREF on node NDc slightly rises.However, isolation instructing signal MLI is driven to the H level andisolation gate circuit 30 is conductive. Therefore, redistribution ofthe charges compensates for the voltage rising on node NDc causedthrough the capacitive coupling, and internal node NDc is keptsubstantially at the level of power supply voltage VDD. In this case,reference voltage MLREF may be kept at the voltage level higher thanpower supply voltage VDD. Since the search operation is performed whilereference voltage MLREF is accurately precharged to the level ofprecharge voltage VML according to precharge instructing signal MLPRE inthe next search cycle, no particular problem in operation occurs.

Reference voltage down instructing signal REFDOWN may be driven to the Hlevel when match amplifier activating signal MAE is in the active stateat the H level. In this state, internal node NDc is not in theelectrically floating state, amplifier circuit 12 rapidly absorbs thevoltage change caused by the capacitive coupling of capacitance elementC2_B.

A next search cycle starts at time T5. Specifically, prechargeinstructing signal MLPRE attains the H level, and match line ML,reference node NDa and internal nodes NDb and NDc are precharged to thelevel of precharge voltage VML.

At time T6, search data is transferred to search lines SL and /SL. Inthis search operation, match line ML is not discharged as it in thematch state. Thus, match line ML is kept at the level of prechargevoltage VML. Therefore, match amplifier 200 holds the voltages ML_MA andMLREF on internal nodes NDb and NDc at the level of precharge voltageVML.

At time T7, isolation instructing signal MLI is driven to the L level,and isolation gate circuit 30 turns non-conductive. Then, referencevoltage down-conversion instructing signal REFDOWN is driven to the Llevel. Through the capacitive coupling of capacitance element C2_B,level of voltage MLREF on internal node NDc lowers. Voltage ML_MA oninternal node NDb is at the level of precharge voltage VML. Therefore, avoltage difference is developed between internal nodes NDb and NDc. Byactivating match amplifier activating signal MAE, amplifier circuit 12differentially amplifies the voltages on nodes NDb and NDc. Accordingly,the voltage ML_MA is driven to the level of power supply voltage VDD,and voltage MLREF is driven to the level of ground voltage. Then, latchinstructing signal LAT is driven to the H level. Responsively, latch 16enters the through state. Output signal ML_OUT of latch 16 attains the Hlevel indicating the match state, and the determination can be performedaccurately.

When the search/determination operation is completed, search lines SLand /SL are driven to the ground voltage level at time T8. Latchinstructing signal LAT is driven to the L level. Responsively, latch 16enters the latch state, and output signal ML_OUT thereof is kept at theH level. Then, match amplifier activating signal MAE is driven to the Llevel. Thereafter, reference voltage down instructing signal REFDOWN isdriven to the H level, and then isolation instructing signal MLI isdriven to the H level. Accordingly, isolation gate circuit 30 turnsconductive to connect internal node NDc to reference voltage node NDa.In this case, isolation gate circuit 30 turns conductive in response tothe rising of reference voltage down instructing signal REFDOWN, tocause the redistribution of charges even when the capacitive coupling ofcapacitance element C2_B raises the voltage level of node NDc.Accordingly, internal node NDc is kept at the ground voltage level.

Through a series of these operations, the search/determination operationis completed. At time T9, the control stands by for a next searchoperation.

In this seventeenth embodiment also, match line precharge instructingsignal MLPRE is driven to and maintained at the H level for apredetermined period (half a clock cycle period), for precharging matchline ML and reference voltage node NDa only at the start of the searchoperation. However, this precharge operation may be performed duringstandby, i.e., when the search operation is not performed (in a periodafter search lines SL and /SL are driven to the ground voltage level).

In the seventeenth embodiment, the sensing operation (amplifyingoperation) is performed in such a state that isolation gate circuit 30is confining the charges on internal nodes NDb and NDc. The loadcapacitances of internal nodes NDb and NDc are smaller than the loadcapacitance of the whole match line ML connected to unit cells UC.Therefore, capacitance element C2_B that lowers the level of voltageMLREF on internal node NDc can have a sufficiently small capacitancevalue. This can reduce an occupation area of match amplifier 200.

Capacitance element C2_A of internal node NDb is arranged for securing acapacitive balance between internal nodes NDb and NDc. Therefore, thecapacitance value of capacitance element C2_A can be merely equal tothat of capacitance element C2_B, and the occupation area thereof can besufficiently small. The load capacitances of nodes NDb and NDc aresmall, and the driving capacitance of amplifier circuit 12 is small sothat the sense operation can be performed fast with low powerconsumption.

The CAM according to the seventeenth embodiment is the same in wholeconstruction as the CAM shown in FIG. 53.

FIG. 57 schematically shows a construction of a portion generatingcontrol signals in the seventeenth embodiment of the invention. Controlsignal generating circuit 220 shown in FIG. 57 differs from controlsignal generating circuit 220 shown in FIG. 54 in the followingconstruction. A match amplifier activating circuit 240 for activatingthe match amplifier according to the output signal of delay circuit 234produces reference voltage down instructing signal REFDOWN instead ofpull-up instructing signal MLUP. This match amplifier activating circuit240 operates in synchronization with clock signal CLK, and is activatedaccording to the output signal of delay circuit 234. Accordingly, matchamplifier activating circuit 240 sequentially activates and deactivatesisolation instructing signal MLI, reference voltage down instructingsignal REFDOWN, sense amplifier activating signal MAE and latchinstructing signal LAT. Reference voltage down instructing signalREFDOWN is the same in waveform as pull-up instructing signal MLUP inthe control signal generating circuit shown in FIG. 54 except for thatthe polarities are inverted. Therefore, match amplifier activatingcircuit 240 in the seventeenth embodiment can be implemented bysubstantially the same construction as match amplifier activatingcircuit 236 shown in FIG. 54. An inverted signal of pull-up instructingsignal MLUP is produced as reference voltage down instructing signalREFDOWN.

For other constructions, control signal generating circuit 220 shown inFIG. 57 is substantially the same as the control signal generatingcircuit shown in FIG. 54. Corresponding portions are allotted the samereference numerals, and description thereof is not repeated.

According to the seventeenth embodiment of the invention, as describedabove, the match amplifier for sensing the voltage level of the matchline confines the match line voltage and the comparison referencevoltage on the internal nodes. This reference voltage level is loweredthrough a capacitive coupling by a voltage level equal to or smallerthan half precharge voltage VIAL, and the sensing operation isperformed. Therefore, in the search/determination operation, a voltagedifference can be developed between the match line and the referencevoltage node, to perform accurately the determining operation. A drivingload of the amplifier circuit in the match amplifier is sufficientlysmall, and the occupation area of the match amplifier can besufficiently small. The sensing operation can be performed fast with lowpower consumption.

Eighteenth Embodiment

FIG. 58 schematically shows a main portion of a CAM according to aneighteenth embodiment of the invention. In the construction of the CAMshown in FIG. 58, match amplifier 200 has a construction different fromthat of match amplifier 200 shown in FIG. 55. Specifically, acapacitance element C3_B is connected to reference voltage node NDa.Capacitance element C3_B lowers the precharge voltage level of referencevoltage node NDa through the capacitive coupling according to alevel-down instructing signal RFDWN. A capacitive element is notprovided for internal nodes NDb band NDc. Match amplifier 200 shown inFIG. 58 is the same in construction as match amplifier 200 shown in FIG.55. The same portions are allotted the same reference numbers, anddescription thereof is not repeated. The arrangement of unit cells UCincluding the CAM cells are the same as that of unit cells UC for matchline ML in the CAM shown in FIG. 55.

In the construction of the CAM shown in FIG. 58, precharge level VML(VML<VDD/2) of reference voltage node NDa is lowered by a certainvoltage level in the search operation. Thereafter, isolation gatecircuit 30 confines the charges of the potential of match line ML and ofthe reference voltage at the lowered level on internal nodes NDb andNDc. Thus, the sensing operation can be performed fast with low powerconsumption.

FIG. 59 is a timing chart representing an operation of the matchamplifier shown in FIG. 58. In the search cycle between times T1 and T4,FIG. 59 shows waveforms corresponding to the search result being themismatch. In the search cycle between times T5 and T8, FIG. 59 showswaveforms corresponding to the search result being the match. Referringto FIG. 59, an operation of match amplifier 200 shown in FIG. 58 willnow be described.

At time T1, the search cycle starts. Precharge instructing signal MLPREis driven to the H level and precharge circuit 210 precharges match lineML and reference voltage node NDa to the level of precharge voltage VML.At this time, isolation gate circuit 30 is conductive (isolationinstructing signal ML is at the H level), and internal nodes NDb and NDcare precharged to the level of precharge voltage VML.

At time T2, match line precharge instructing signal MLPRE attains the Llevel to start the search operation. Search lines SL and /SL are drivenaccording to the search data. In the case of mismatch, match line ML isdischarged. At this time, level-down instructing signal REFDWN is drivento the L level according to the start of the search operation.Therefore, through the capacitive coupling of capacitance element C3_B,the voltage level of reference voltage node NDa lowers from prechargevoltage VML. The quantity of the voltage level drop of reference voltagenode NDa is equal to or smaller than half a difference between prechargevoltage VML and ground voltage GND. Therefore, even when the level ofvoltage MLREF lowers, the voltage MLREF is higher than the voltage ML_MAof match line ML.

The changes in voltage on match line ML and reference voltage node NDaare transmitted to internal nodes NDb and NDc via isolation gate circuit30.

At time T3, isolation instructing signal MLI is driven to the L level,and isolation gate circuit 30 turns non-conductive. When isolation gatecircuit 30 changes to the non-conductive state, level-down instructingsignal REFDWN is driven to the H level. Through the capacitive couplingof capacitance element C3_B, the voltage level of reference voltage nodeNDa rises. However, isolation gate circuit 30 is non-conductive, andvoltage MLREF on internal node NDc holds the level lowered at time T2.

Then, match amplifier activating signal MAE is made active. Amplifiercircuit 12 is made active to amplify differentially voltages MLREF andML_MA on internal node NDb and NDc. Since the search result is themismatch state, the voltage MLREF is driven to the level of power supplyvoltage VDD, and voltage ML_MA is driven to the ground voltage level.Then, latch instructing signal LAT is driven to the H level, and latch16 enters the through state. Responsively, output signal ML_OUT of latch16 is driven to the L level to indicate the mismatch state.

When the search operation is completed at time T4, latch instructingsignal LAT is driven to the L level, and then match amplifier activatingsignal MAE is driven to the L level. Accordingly, latch 16 enters thelatch state, and amplifier circuit 12 is made inactive. After thedeactivation of the match amplifier, isolation instructing signal MLI isdriven to the H level to turn isolation gate circuit 30 conductive.

Match line ML is already discharged to the ground voltage level, andvoltage ML_MA on internal node NDb is kept at the level of groundvoltage even when isolation gate circuit 30 turns conductive. Internalnode NDc is connected to reference voltage node NDa. The charges areredistributed between internal node NDb and reference voltage node NDa.In this case, the voltage level changes according to the ratio betweenthe parasitic capacitance (the capacitance of capacitance element C3_Band the interconnection capacitance) of reference voltage node NDa andthe parasitic capacitance (the gate capacitance of the transistors ofamplifier circuit 12 and the coupling capacitance) of internal node NDc.FIG. 59 shows a state in which voltage MLREF is kept at the level ofpower supply voltage VDD. In the case where the capacitive couplingincreases the voltage level of reference voltage node NDa when isolationgate circuit 30 is non-conductive, the quantity of voltage rising islarger than the quantity of voltage lowering. At the time of voltagerising, the parasitic capacitance coupled to reference voltage node NDais smaller than that at the time of voltage lowering. Therefore, evenwhen isolation gate circuit 30 is made conductive to redistribute thecharges between nodes NDc and NDa, the quantity of changes in voltage issmall, and it can be supposed that nodes NDa and NDc are kept at thepower supply voltage level.

Even when voltage MLREF changes to the voltage level between prechargevoltage VML and power supply voltage VDD after isolation gate circuit 30is made conductive, no particular problem occurs (because referencevoltage node NDa is precharged to the level of precharge voltage VML atthe start of the next search cycle).

The next search cycle starts at time T5, and precharge instructingsignal MLPRE is driven to the active state. Responsively, MOStransistors 211 and 212 of precharge transistor 210 precharge match lineML and reference voltage node NDa to the level of precharge voltage VML.

At time T6, the search data is transmitted to search lines SL and /SLand the voltage levels thereof change. In the case of the match state,the discharging path for match line ML is not present. Therefore, matchline ML is kept at the level of precharge voltage VML. Also, at thestart of the search operation, level-down instructing signal REFDWN isdriven to the L level. Responsively, through the capacitive coupling ofcapacitance element C3_B, the voltage level of reference voltage nodeNDa drops. The quantity of voltage drop of reference voltage node NDa isequal to a level between precharge voltage VML and ground voltage GND.Isolation gate circuit 30 is conductive and transmits the voltage levelsof match line ML and reference voltage node NDa to internal nodes NDband NDc, respectively.

At time T7, isolation instructing signal MLI is driven to the L level insynchronization with the rising of clock signal CLK. Responsively,isolation gate circuit 30 is made non-conductive, so that the chargesare confined on internal nodes NDb and NDc. After isolation gate circuit30 turns into the non-conductive state, level-down instructing signalREFDWN is driven to the H level. In this case, through the capacitivecoupling of capacitance element C3_B, the voltage level of referencevoltage node NDa changes. However, isolation gate circuit 30 isnon-conductive, and the level of voltage MLREF on internal node NDc doesnot change.

Then, sense amplifier activating signal MAE is made active, and thevoltages ML_MA and MLREF on internal nodes NDb and NDc aredifferentially amplified. Accordingly, voltage ML_MA is driven to thelevel of power supply voltage VDD, and voltage MLREF is driven to theground voltage level.

Then, latch instructing signal LAT is driven to the H level to set latch16 to the through state. Responsively, output signal ML_OUT of latch 16is driven to the H level indicative of the match state.

At time T8, the determining operation is completed, and latchinstructing signal LAT is driven to the L level to set latch 16 to thelatch state. Output signal ML_OUT of latch 16 keeps the H level.Thereafter, sense amplifier activating signal MAE is made inactive, andthen isolation instructing signal MLI is made inactive (or driven to theH level). Accordingly, isolation gate circuit 30 turns conductive tocouple internal nodes NDb and NDc to match line ML and reference voltagenode NDa, respectively. In this case, match line. ML is at the level ofprecharge voltage VML, and the level of voltage ML_MA lowers accordingto the capacitance ratio between internal node NDb and match line ML.FIG. 59 does not clearly show this voltage change.

The level of voltage MLREF is likewise set according to the capacitanceratio between internal node NDc and reference voltage node NDa. However,this change in voltage MLREF is also not shown.

Even when isolation gate circuit 30 turns conductive to change thevoltage levels of nodes NDa, NDb and NDc, no particular problem occursin connection with the next search operation. This is because accordingto precharge instructing signal MLPRE, match line ML, reference voltagenode NDa and internal nodes NDb and NDc are precharged to the level ofprecharge voltage VML at the start of the next search cycle.

In the construction according to the eighteenth embodiment of theinvention, a capacitance element is not provided for internal nodes NDband NDc. Therefore, internal nodes NDb and NDc do not require acapacitance element for maintaining a capacitive balance. This furtherreduce an occupation area of match amplifier 200. Internal nodes NDb andNDc are not provided with a capacitance element (except for parasiticcapacitances). Therefore, it is possible to reduce the capacitance ofthe nodes on which the voltage full-swings in the amplifying operationof amplifier circuit 12, and the current consumption can be reduced.

The voltage lowering of reference voltage node NDa caused by downinstructing signal REFDWN is performed concurrently with thetransmission of the search data to search lines SL and /SL. However, thesense operation by amplifier circuit 12 can be performed immediatelyafter the transition of isolation gate circuit 30 to the non-conductivestate. Thereby, match amplifier 200 can start the sensing operation at afaster timing.

FIG. 60 schematically shows a construction of a portion of the controlsignals of the CAM shown in FIG. 58. In FIG. 60, control signalgenerating circuit 220 differs from the control signal generatingcircuit shown in FIG. 57 in the following construction. A matchamplifier activating circuit 250 sequentially activates or deactivatesisolation instructing signal MLI, sense amplifier activating signal MAEand latch instructing signal LAT according to the output signal of delaycircuit 234 and clock signal CLK.

A down activating circuit 252 drives reference voltage down instructingsignal REFDWN to the L and H levels according to isolation instructingsignal MLI and search operation activating signal SLEN, respectively.Specifically, down activating circuit 252 drives the down instructingsignal REFDWN to the L level when search operation activating signalSLEN becomes active to start the search operation. When isolationinstructing signal MLI attains the L level, down activating circuit 252drives the down instructing signal REFDWN to the H level. Downactivating circuit 252 can be implemented using, e.g., a set/resetflip-flop.

Match amplifier activating circuit 250 activates or deactivates,according to a delay enable signal applied from delay circuit 234,isolation instructing signal MLI, sense amplifier activating signal MAEand latch instructing signal LAT in a predetermined sequence insynchronization with clock signal CLK after elapsing of one clock cyclefrom the application of search operation command CMD.

Other constructions of control signal generating circuit 220 shown inFIG. 60 are the same as those of the control signal generating circuitshown in FIG. 57. Corresponding portions are allotted the same referencenumerals, and description thereof is not repeated.

According to the eighteenth embodiment of the invention, as describedabove, the capacitive coupling lowers the voltage level of the referencevoltage node that produces the comparison reference voltage of the matchline, and then the voltage level of the match line is sensed with thecharges confined. Therefore, it is no necessary to arrange a capacitanceelement for the sense nodes of the amplifier circuit performing thesense operation, and the occupation area can be small. Further, the loadcapacitance of the amplifier circuit performing the sense operation canbe reduced, and the fast sensing and the low power consumption can beachieved.

Nineteenth Embodiment

FIG. 61 shows a main portion of a CAM according to a nineteenthembodiment of the invention. FIG. 61 representatively shows unit cellsUC corresponding to one match line ML as well as corresponding matchamplifier 200. Match amplifiers each being substantially the same inconstruction as match amplifier 200 shown in FIG. 61 are arrangedcorresponding to match lines ML, respectively.

Match amplifier 200 shown in FIG. 61 differs from match amplifier 200shown in FIG. 58 in the following construction. For reference voltagenode NDa, there is arranged a voltage down circuit 260 that lowers thevoltage level of precharge voltage VML on reference voltage node NDaaccording to level-down instructing signal RFDWN. Voltage down circuit260 includes a capacitance element C4, and N-channel MOS transistors 261and 262 controlling the charging/discharging of capacitance element C4.

N-channel MOS transistor 262 discharges one electrode node (NDd) ofcapacitance element C4 to the ground voltage level according toprecharge instructing signal MLPRE. MOS transistor 261 couples the oneelectrode node NDd of capacitance element C4 to reference voltage nodeNDa according to level-down instructing signal RFDWN. MOS transistor 212of precharge circuit 210 precharges reference voltage node NDa toprecharge voltage VML. The quantity of charges accumulated on referencevoltage node NDa is determined by the parasitic capacitance of referencevoltage node NDa and the voltage level of precharge voltage VML.Capacitance element C4 for precharging node NDd to the ground voltagelevel is coupled to reference voltage node NDa, and the charges movebetween reference voltage node NDa and capacitance element C4 to lowerprecharge voltage VML on reference voltage node NDa according to thecapacitance values of parasitic capacitances of reference node NDa andinternal node NDc as well as the capacitance value of capacitanceelement C4.

Other constructions of match amplifier 200 shown in FIG. 61 as the sameas those of match amplifier 200 shown in FIG. 58. Corresponding portionsare allotted the same reference numerals, and description thereof is notrepeated. The construction and arrangement of the unit cells coupled tomatch line ML are the same as those shown in FIG. 58. Correspondingportions are allotted the same reference numerals, and descriptionthereof is not repeated.

FIG. 62 is a timing chart representing the operation of match amplifier200 shown in FIG. 61. Referring to FIG. 62, the operation of matchamplifier 200 shown in FIG. 61 will now be described.

In the search cycle from time T1 to time T4, the mismatch determinationis performed. In the search cycle from time T5 to time T9, the matchdetermination is performed.

When the search cycle starts at time T1, precharge instructing signalMLPRE attains the H level. Responsively, precharge circuit 210precharges match line ML and reference voltage node NDa to the voltagelevel of precharge voltage VML (of VDD/2 or lower). At this time,isolation gate circuit 30 is conductive and internal nodes NDb and NDcare precharged to the level of precharge voltage VML.

In voltage down circuit 260, MOS transistor 262 is turned on to couplenode NDd to the ground node according to precharge instructing signalMLPRE so that the accumulated charges in capacitance element C4 aredischarged. When the search operation starts at time T2, search lines SLand /SL are driven according to the search data. In the mismatch state,match line ML is discharged to have a lowered voltage level. At thistime, level-down instructing signal RFDWN is driven to the H level.Responsively, MOS transistor 261 is turned in voltage down circuit 260,and couples one electrode (NDd) of capacitance element C4 to referencevoltage node NDa. Accordingly, capacitance element C4 is charged withthe charges accumulated on reference voltage node NDa. Through themovement of the charges, the voltage levels of nodes NDa and NDc lower.The quantity of the voltage lowering is equal to a level of the voltageVML/2 intermediate between precharge voltage VML and ground voltage GND,and nodes NDa and NDc attain the voltage levels between the groundvoltage and precharge voltage VML.

Internal node NDb is coupled to match line ML via isolation gate circuit30, and the level of voltage ML_MA lowers with the voltage lowering ofmatch line ML.

When clock signal CLK rises at time T3, isolation instructing signal MLIis driven to the L level and responsively, isolation gate circuit 30turns non-conductive. Then, level-down instructing signal RFDWN isdriven to the L level to turn off MOS transistor 261. In this state,charges corresponding to voltages ML_MA and MLREF are confined oninternal nodes NDb and NDc.

Then, match amplifier activating signal MAE is made active and thevoltages ML_MA and MLREF on internal nodes NDb and NDc are amplifieddifferentially. In this differentially amplifying operation, accordingto the capacitance value of capacitance element C4, the voltages onnodes NDa and NDc lower to the voltage between precharge voltage VML andground voltage GND. Thus, the voltage difference that can be sensed byamplifier circuit 12 is developed between nodes NDc and NDb, and thesensing operation can be accurately performed. After the completion ofthe sensing operation, latch instructing signal LAT attains the H level,and latch 16 produces output signal ML_OUT at the L level to indicatethe mismatch state.

When the search/determination operation ends, the driving of searchlines SL and /SL ends, and search lines SL and /SL are driven to theground voltage level. Latch instructing signal LAT attains the L level,and match amplifier activating signal MAE becomes inactive. Latch 16enters the latch state, and output signal ML_OUT is kept at the L level.Amplifier circuit 12 keeps the inactive state, and nodes NDb and NDcattain the floating state at the levels of ground voltage and powersupply voltage to, respectively.

Thereafter, isolation instructing signal MLI is driven to the H level sothat isolation gate circuit 30 is made conductive. Responsively,internal nodes NDb and NDc are connected to match line ML and referencevoltage node NDa, respectively. Match line ML is at the ground voltagelevel, and voltage ML_MA is kept at the level of ground voltage.Level-down instructing signal RFDWN is at the L level, reference voltagenode NDa is in the floating state and voltage MLREF on internal node NDcis kept substantially at the level of power supply voltage VDD. Throughmovement of charges, nodes NDc and NDa may be driven to the voltagelevels lower than power supply voltage VDD.

One electrode node (NDd) of capacitance element C4 is kept at thevoltage level attained through the charging in the cycle starting attime T2.

The search cycle starts at time T5 again, and precharge instructingsignal MLPRE is activated (or driven to the H level). Accordingly, matchline ML and internal node NDb are precharged to the level of prechargevoltage VML. Reference voltage node NDa and internal node NDc areprecharged to the level of precharge voltage VML. The one electrode node(NDd) of capacitance element C4 is discharged to the ground voltagelevel.

The search operation starts at time T6. In this case, the voltage levelsof search lines SL and /SL change according to the search data. In thematch state, match line ML is not discharged, and match line ML is keptat the level of precharge voltage VML. Concurrently with this searchoperation, level-down instructing signal RFDWN attains the H level, andone electrode node NDd of capacitance element C3 is coupled to referencevoltage node NDa via MOS transistor 261. Responsively, charges move tolower the voltage levels of reference voltage node NDa and internal nodeNDc. The quantity of this lowering of voltage MLREF is equal to avoltage level intermediate between precharge voltage VML and groundvoltage GND.

At time T7, isolation instructing signal MLI attains the L level insynchronization with the rising of clock signal CLK, and subsequentlylevel-down instructing signal RFDWN is driven to the L level.Responsively, one electrode node NDd of capacitance element C4 isisolated from reference voltage node NDa. Isolation gate circuit 30 ismade non-conductive to confine the charges on internal nodes NDb andNDc. In this case, the precharge voltage on internal node NDb is at thelevel of VML, and voltage MLREF of internal node NDc is at a levelintermediate between precharge voltage VML and ground voltage GND.Therefore, a sufficient voltage level is developed between nodes NDb andNDc.

Then, match amplifier activating signal MAE is activated so thatamplifier circuit 12 differentially amplifies the voltages ML_MA andMLREF. Thereby, voltage ML_MA on internal node NDb and voltage MLREF oninternal node NDc are driven to and latched at power supply voltage VDDand the ground voltage level, respectively.

Then, latch instructing signal LAT is driven to the H level.Responsively, latch 16 enters the through state, and produces outputsignal ML_OUT at the H level.

At time T8, the search/determination operation is completed, and latchinstructing signal LAT and match amplifier activating signal MAE aresequentially driven to the L level. Amplifier circuit 12 is madeinactive, and internal nodes NDb and NDc attain the floating state atthe levels of power supply voltage VDD and ground voltage GND,respectively.

Thereafter, isolation instructing signal MLI rises to the H level toturn isolation gate circuit 30 conductive. Responsively, internal nodeNDb is coupled to match line ML, and internal node NDc is coupled toreference voltage node NDa. Match line ML has a capacitance larger thana parasitic capacitance of node NDb. Therefore, when isolation gatecircuit 30 does not have a decoupling function, the charge distributionis performed between match line ML at the level of precharge voltage VMLand internal node NDb at the power supply voltage level according to thecapacitance values thereof. In this case, the voltage level of internalnode NDb lowers from the power supply voltage level, but FIG. 62 doesnot clearly show this voltage change. When the charge distributionoccurs, match line ML attains a voltage level higher than prechargevoltage VML, and voltage ML_MA on internal node NDb attains the levellower than power supply voltage VDD. When the charges move, the voltagelevel of match line ML finally becomes equal to the level of voltageML_MA on internal node NDb.

However, where the difference between the voltage level of isolationinstructing signal MLI and the threshold voltage of isolation gates TGaand TGb of isolation gate circuit 30 is a level slightly higher thanprecharge voltage VML, these isolation gates TGa and TGb operate asdecoupling transistors. In this case, match line ML is kept at the levelof precharge voltage VML, and voltage ML_MA on internal node NDb is keptat the level of power supply voltage VDD.

After the end of search/determination operation, level-down instructingsignal RFDWN is at the L level, and one electrode node NDd ofcapacitance element C4 hodls the charged state.

In this nineteenth embodiment, capacitance element C4 in the voltagedown circuit lowers the voltage level of reference voltage node NDathrough the charge redistribution. Therefore, the lowered voltage levelof reference voltage node NDa does not depend on the voltage level ofpower supply voltage VDD, and reference voltage node NDa can be set tothe voltage level that is set by the stable capacitance value ofcapacitance element C4. (Instructing signal RFDWN or REFFDOWN has theamplitude of the level of power supply voltage VDD.) Therefore, thesensing operation can be performed stably.

Specifically, when the voltage level of comparison reference voltageMLREF changes depending on power supply voltage VDD in the searchoperation, there causes a change in voltage difference between internalnodes NDb and NDc attained when the voltage level of match line ML isconfined. Thus, variations occur in voltage difference between the sensenodes of amplifier circuit 12, and a sense margin lowers so that it maypossibly be impossible to ensure the accurate sensing operation. Thevoltage level of comparison reference voltage MLREF is set byredistributing the charges, using capacitance element C4. The quantityof the voltage change depends on the capacitance value of capacitanceelement C4 and the capacitance value of the parasitic capacitances ofnodes NDa and NDc, and does not depend on the amplitude of thelevel-down instructing signal, i.e., the power supply voltage. Thereby,at the start of the amplifying operation of amplifier circuit 12, thevoltage level of node NDc can be accurately set so that the fast andstable sense operation can be achieved.

For the construction of the circuit for generating the control signalsin the nineteenth embodiment, the construction of control signalgenerating circuit 220 shown in FIG. 60 can be used. The control signalRFDWN is produced by inverting the signal polarities of down instructingsignal REFDWN applied from voltage down activating circuit 252 shown inFIG. 60.

In this nineteenth embodiment, precharge instructing signal MLPRE may bemade active in the standby state during which the search operation isnot performed.

According to the nineteenth embodiment of the invention, as describedabove, the level of the comparison reference voltage is set by thecharge redistribution using the capacitance element. Therefore, thevoltage level of the comparison reference voltage can be accurately setin the sensing operation, and the stable sensing operation can beensured. The sensing operation is performed in the charge confiningscheme, and the effects similar to those of the eighteenth embodimentcan be also achieved.

Twentieth Embodiment

FIG. 63 schematically shows a main portion of a CAM according to atwentieth embodiment of the invention. In FIG. 63, the CAM cell array isprovided with n match lines ML[i]-ML[i+n]. A plurality of unit cells UCare connected to each match line, but FIG. 63 representatively showsonly unit cells UC connected to match line ML[i].

Match amplifiers 200 i-200(i+n) are arranged corresponding to matchlines ML[i]-ML[i+n], respectively. Each of match amplifiers 200i-200(i+n) includes a precharge circuit 270 for precharging acorresponding match line, amplifier circuit 12, and latch 16 forlatching an output of amplifier circuit 12. Similarly to the embodimentsdescribed heretofore, amplifier circuit 12 amplifies the potentials oninternal nodes NDb and NDc in response to match amplifier activatingsignal MAE. Latch 16 enters the through state and latch state accordingto latch instructing signal LAT, and latches the output signal ofcorresponding amplifier circuit 12.

Precharge circuit 270 includes only precharge transistor 211 forprecharging a corresponding match line ML (ML[i]-ML[i+n]). Matchamplifiers 200 i-200(i+n) each are not provided with a prechargingtransistor (212) individually.

In each of match amplifiers 200 i-200(i+n), isolation gate circuit 30 isarranged at the stage preceding amplifier circuit 12. In FIG. 63,amplifier circuit 12 in each of match amplifiers 200 i-200(i+n) isrepresented by a circuit having complementary and positive control nodesand output nodes producing complementary signals. Amplifier circuit 12in each match amplifier is the same in construction. Amplifier circuit12 of each of match amplifiers 200(i+1)-200(i+n) receives acomplementary match amplifier activating signal ZMAE on itscomplementary control node via the inverter.

A main voltage down circuit 300 is provided commonly to and to be hsaredby match amplifiers 200 i-200(i+n). Main voltage down circuit 300includes a capacitance element C5 and MOS transistors 301-303. MOStransistor 301 precharges an output node MND0 to the level of prechargevoltage VML according to precharge instructing signal MLPRE. MOStransistor 302 couples output node MND0 to one electrode node (MND1) ofcapacitance element C5 according to level-down instructing signal RFDWN.MOS transistor 303 discharges one electrode node MND1 of capacitanceelement C5 to the ground voltage level according to prechargeinstructing signal MLPRE. Output node MND0 of main voltage down circuit300 is commonly connected to reference voltage nodes NDa of matchamplifiers 200 i-200(i+n). Capacitance element C5 has a capacitancevalue equal to or smaller than half a combined parasitic capacitance ofthese reference voltage nodes NDa, and lowers the voltages by a voltagelevel (VML/2) intermediate between precharge voltage VML andground-voltage GND, for these match amplifiers 200 i-200(i+n).

FIG. 63 is a timing chart representing the operations for the search anddetermination of the CAM, and this timing chart is the same as thatshown in FIG. 62. For the control circuit for producing various controlsignals, the construction substantially the same as the control signalgenerating circuit shown in FIG. 60 can be utilized. Therefore, the CAMaccording to the twentieth embodiment of the invention performssubstantially the same search and determination operations as those inthe nineteen the embodiment of the invention, and the descriptionthereof is not repeated.

In the construction of the CAM shown in FIG. 63, the plurality of matchamplifiers 200 i-200(i+n) share the circuit for lowering comparisonreference voltage MLREF to a voltage level intermediate betweenprecharge voltage VML and ground voltage GND in the operation of sensingthe voltage level of the match line. Thereby, as compared with theconstruction in which an individual and separate voltage down circuit isarranged for each amplifier, the occupation area can be reduced. Also,the similar effects as those of the eighteenth embodiment can beachieved.

Twenty-First Embodiment

FIG. 64 schematically shows a construction of a CAM according to atwenty-first embodiment of the invention. The CAM shown in FIG. 64differs from the CAM shown in FIG. 63 in the following construction. Ineach of match amplifiers 200 i-200(i+n), an N-channel MOS transistor 310rendered conductive in response to an equalize instructing signal MLEQis arranged between internal nodes NDb and NDc. Other constructions ofthe CAM shown in FIG. 64 are the same as those of the CAM shown in FIG.63. Corresponding portions are allotted the same reference numerals, anddescription thereof is not repeated.

By using equalizing MOS transistors 310 in match amplifiers 200i-200(i+n), it is possible to reduce the time required for restoringmatch line ML, internal voltage ML_MA and comparison reference voltageMLREF to precharge voltage VML.

FIG. 65 is a timing chart representing the search operation of the CAMshown in FIG. 64. Referring to FIG. 65, the operation of the CAM shownin FIG. 64 will now be described.

In the timing chart of FIG. 65, the precharge operation is performedduring the standby state. Thus, precharge instructing signal MLPRE iskept active except a period in which the search operation is performedand search lines SL and /SL are driven according to the search data.

In the timing chart of FIG. 65 also, the search/determination cycle fromtime T1 to time T4 is performed in the case of the mismatch state, andthe match state is sensed in the search/determination cycle from time T5to time T8.

Before time T1, precharge instructing signal MLPRE is at the H level,and match line ML and internal nodes NDb and NDc are precharged to thelevel of precharge voltage VML. In this state, precharge instructingsignal MLPRE causes main voltage down circuit 300 to discharge oneelectrode node MND1 of capacitance element C5 to the ground voltagelevel.

Also, equalize instructing signal MLEQ is at the H level, and equalizingMOS transistor 310 is conductive. Therefore, the voltages ML_MA andMLREF are equalized to the level of precharge voltage VML.

At time T1, the search cycle starts so that match line prechargeinstructing signal MLPRE is deactivated (driven to the L level), andequalize instructing signal MLEQ is also deactivated (driven to the Llevel). Accordingly, the precharge operation and equalize operation onmatch line ML (ML[i]-ML[i+n]) ends. Match line ML attains theelectrically floating state at the level of precharge voltage VML.

Then, search lines SL and /SL are driven according to the search data.When the search result indicates the mismatch, match line ML isdischarged to the ground voltage level. Concurrently with this searchoperation, level-down instructing signal RFDWN attains the H level, andone electrode node MND1 of capacitance element C5 is coupled toreference voltage node NDa in each of match amplifiers 200 i-200(i+n)via output node MND0. Responsively, the voltage level on referencevoltage node NDa lowers in each of match amplifiers 200 i-200(i+n).Since isolation gate circuit 30 is still conductive, the voltage MLREFof internal node NDc of each of match amplifiers 200 i-200(i+n) lowersfrom the level of precharge voltage VML.

When a voltage difference between voltage ML_MA on match line ML andcomparison reference voltage MLREF sufficiently increases, isolationinstructing signal MLI is driven to the L level at time T3. Accordingly,isolation gate circuit 30 is made non-conductive in each of matchamplifiers 200 i-200(i+n), and confines the charges on internal nodesNDb and NDc.

When isolation instructing signal MLI attains the L level, level-downinstructing signal RFDWN also attains the L level so that MOS transistor302 in main voltage down circuit 300 is turned off. Thereby, oneelectrode node MND1 of capacitance element C5 attains the voltage levelset through the redistribution of the charges.

At time T3, the voltage level of match line ML is already driven to thevoltage level corresponding to the search result. Therefore, the searchoperation ends at this time point, and search lines SL and /SL aredeactivated (driven to the ground voltage level).

In this state, precharge instructing signal MLPRE is made active againand match lines ML are precharged to the level of precharge voltage VML.Main voltage down circuit 300 discharges one electrode node MND1 ofcapacitance element C5 via MOS transistor 303 to the ground voltagelevel. MOS transistor 301 in main voltage down circuit 300 charges eachreference voltage node NDa to the level of precharge voltage VML.

After isolation instructing signal MLI is driven to the L level, matchamplifier activating signal MAE is activated, and then latch instructingsignal LAT is driven to the H level. Accordingly, each of matchamplifiers 200 i-200(i+n) performs the sensing operation by amplifiercircuit 12. In the match amplifier for the mismatch state, latch 16produces output signal ML_OUT at the L level. At subsequent time T4,latch instructing signal LAT is driven to the L level according to thefalling of clock signal CLK, and then sense amplifier activating signalMAE is driven to the L level, to complete the sensing operation.

Thereafter, isolation instructing signal MLI attains the H level, andisolation gate circuit 30 in each of match amplifiers 200 i-200(i+n) ismade conductive. Accordingly, internal nodes NDb of match amplifiers 200i-200(i+n) is coupled to corresponding match lines ML[i]-ML[i+n], andreference voltage nodes NDa are coupled to corresponding internal nodesNDc. At this time, equalize instructing signal MLEQ attains the H levelto turn on equalizing MOS transistor 310. In each of match amplifiers200 i-200(i+n), therefore, voltages ML_MA and MLREF on respective nodesNDb and NDc already driven to the levels of power supply voltage VDD andground voltage GND are rapidly driven to the level of precharge voltageVML.

When the search operation starts again at time T5, match line prechargeinstructing signal MLPRE attains the L level, equalize instructingsignal MLEQ attains the L level and match line ML attains theelectrically floating state at the level of precharge voltage VML. Leveldown instructing signal RFDWN attains the H level, and one electrodenode MND1 of capacitance element C5 in main voltage down circuit 300 iscoupled to each of reference voltage nodes NDa of match amplifiers 200i-200(i+n). Thereby, voltage MLREF on node NDc in each of matchamplifiers 200 i-200(i+n) is driven to the level lower than prechargevoltage VML.

The search operation is performed, and the voltage levels of searchlines SL and /SL are driven to the levels corresponding to the searchdata. In the match state, match line ML is at the level of prechargevoltage VML. Through the voltage down operation, voltage MLREF is keptat the level intermediate between precharge voltage VML and groundvoltage GND, and a sufficient potential difference is already ensuredbetween the sense nodes of amplifier circuit 12.

At time T7, the search operation is completed, and then the determiningoperation is performed. Thus, isolation instructing signal MLI is drivento the L level, and isolation gate 30 in each of match amplifiers 200i-200(i+n) is made non-conductive.

Accordingly, match lines ML[i]-ML[i+n] are isolated from nodes NDb ofcorresponding match amplifiers 200 i-200(i+n), respectively. In thisstate, the search operation is completed, and search lines SL and /SLare driven to the ground voltage level.

When isolation instructing signal MLI is driven to the L level,level-down instructing signal RFDWN is driven to the L level again, andone electrode node MND1 of capacitance element C5 is isolated fromoutput node MND0 of main voltage down circuit 300.

In this state, match amplifier activating signal MAE is made active, andamplifier circuit 12 performs the differential amplification. Then,latch instructing signal LAT is driven to the H level, and latch 16 isset to the through state, to drive output signal ML_OUT to the H levelindicative of the match state.

Concurrently with this sensing operation, match line prechargeinstructing signal MLPRE attains the H level again, and match line ML isdriven to the level of precharge voltage VML. In main voltage downcircuit 300, one electrode node MND1 of capacitance element C5 isdischarged to the ground voltage level via MOS transistor 303. In eachof match amplifiers 200 i-200(i+n), isolation gate circuit 30 isnon-conductive, and this precharging operation exerts no adverse effecton the amplifying operation.

When the voltage level of output signal ML_OUT of latch 16 is thendefinite, the search operation ends at time T8. Specifically, latchinstructing signal LAT is driven to the L level, and then matchamplifier activating signal MAE is deactivated. Thereafter, isolationinstructing signal MLI attains the H level, and isolation gate circuit30 in each of match amplifiers 200 i-200(i+n) is made conductive.Accordingly, match lines ML[i]-ML[i+n] are coupled to internal nodes NDbof corresponding match amplifiers 200 i-200(i+n), respectively. In eachof match amplifiers 200 i-200(i+n), internal node NDc is coupled toreference voltage node NDa.

MOS transistor 301 of main voltage down circuit 300 has prechargedreference voltage node NDa to the level of precharge voltage VMLaccording to precharge instructing signal MLPRE. Therefore, in each ofmatch amplifiers 200 i-200(i+n), through the movement of charges, thevoltage levels of internal nodes NDb and NDc change. In this operation,match line equalize instructing signal MLEQ attains the H level, andequalizing MOS transistor 310 is turned on to drive fast the voltagelevels of nodes NDb and NDc to the level of precharge voltage VML.

At time T9, the determination cycle is completed, and the standby statestarts.

In each of match amplifiers 200 i-200(i+n), as described above, theinternal nodes are equalized using equalizing MOS transistor 310.Concurrently with the sensing operation, the match line and referencevoltage node NDa are precharged while keeping the isolation gate circuitin the non-conductive state. Therefore, the internal nodes of matchamplifiers 200 i-200(i+n) and the match lines can be precharged fast.Consequently, it is possible to reduce the time required for prechargingthe internal nodes of match amplifiers 200 i-200(i+n) and to reduce thelength of the cycle period of the search/determination, and the fastsearch/determination can be achieved.

FIG. 66 schematically shows a construction of a portion for generatingcontrol signals of the CAM shown in FIG. 64. In FIG. 66, a controlsignal generating circuit 400 includes a command decoder 402 fordecoding command CMD to produce search operation instruction ENaccording to a result of the decoding, and a delay circuit 408 fordelaying search operation instruction EN. Command decoder 402 takes incommand CMD at the rising of clock signal CLK, for example and decodesthe command thus taken in. Delay circuit 408 delays search operationinstruction EN applied from command decoder 402 by one cycle period ofclock signal CLK.

Control signal generating circuit 400 further includes a prechargeactivating circuit 406 for producing match line precharge instructingsignal MLPRE, a search line drive activating circuit 404 for producingsearch operation activating signal SLEN and a match amplifier activatingcircuit 410 for producing signals MLI, MAE and LET.

When search operation instruction EN applied from command decoder 402 isactive, search line drive activating circuit 404 activates searchoperation activating signal SLEN for one cycle period of clock signalCLK. A search data input circuit 401 is enabled according to theactivation of search operation activating signal SLEN, and drives thesearch line according to search data SD when made active.

Precharge activating circuit 406 deactivates precharge instructingsignal MLPRE in response to the activation of search operationinstruction EN applied from command decoder 402, and activates prechargeinstructing signal MLPRE in response to the deactivation of searchoperation activating signal SLEN applied from search line driveactivating circuit 404.

Match amplifier activating circuit 410 sequentially activates anddeactivates isolation instructing signal MLI, sense amplifier activatingsignal MAE and latch instructing signal LAT according to the outputsignal of delay circuit 408.

Control signal generating circuit 400 further includes a voltage downactivating circuit 412 for producing level-down instructing signalRFDWN, and an equalize activating circuit 414 for producing equalizeinstructing signal MLEQ. Voltage down activating circuit 412 deactivates(drives to the L level) level-down instructing signal RFDWN according tothe falling of isolation instructing signal MLI provided from matchamplifier activating circuit 410, and drives the level-down instructingsignal RFDWN to the H level in response to the activation of searchoperation instruction EN.

Equalize activating circuit 414 drives equalize instructing signal MLEQto the H level in response to the rising of isolation instructing signalMLI, and drives equalize instructing signal MLEQ to the L levelaccording to the activation of search operation instruction EN.

According to the twenty-first embodiment of the invention, as describedabove, the sense nodes confining the charges are equalized duringstandby in each match amplifier. Therefore, when another searchoperation starts after completion of a search operation, the match linesand the sense nodes (internal nodes NDb and NDc) can be driven fast tothe level of predetermined precharge voltage VML. Thus, the sensingoperation of each match amplifier can be started at a faster timing, andthe determination period can be reduced. Further, similar ffects asthose of the twentieth embodiment can be achieved also.

The construction in which the equalize transistors are provided for thesense nodes (internal nodes NDb and NDc) of amplifier circuit 12 may beapplied to the constructions in the sixteenth to nineteenth embodimentsshown in FIG. 51 and later.

The invention can be applied to the content addressable memory capableof searching the storage information according to the search data, todetermine the match/mismatch with the storage information. Inparticular, when the invention can be used for the construction in whichan IP address is decoded for routing a transfer path in a communicationrouter handling search data of a large bit width, the router can beachieved that can reduce a footprint and power consumption.Alternatively, the content addressable memory according to the inventionmay be used in a circuit construction for determining cache miss/hit ina cache controller or the like.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

1-39. (canceled)
 40. A content addressable memory comprising: aplurality of entries each having a plurality of content addressablememory cells; a plurality of match lines, arranged corresponding to therespective entries, each coupled to the content addressable memory cellsin a corresponding entry; a search data bus coupled in parallel to theentries, for transferring search data in parallel to the entries; and aplurality of match amplifiers, arranged corresponding to the respectivematch lines, each including (i) a precharge circuit for precharging acorresponding match line to a precharge voltage level, (ii) an amplifiercircuit having a first node receiving a voltage on the correspondingmatch line and a second node receiving a sense reference voltageproduced by changing a voltage at the precharge voltage level throughuse of a capacitance element, comparing the voltages on the first andsecond nodes and producing a signal indicating a result of comparison,and (iii) an isolation gate for confining charges on the first andsecond nodes before activation of said amplifier circuit, wherein saidprecharge circuit further precharges a reference voltage line coupled tosaid second node via said isolation gate to the precharge voltage level,and each match amplifier further includes a capacitance element arrangedon said reference voltage line, and performing a charge pump operationaccording to a voltage down instructing signal to lower the voltagelevel of said reference voltage line, said voltage down instructingsignal being activated before isolating by said isolation gate aftercompletion of said precharge operation.